MAHI  B.H.  MlCH^L. 


I 


Digitized  by  the  Internet  Archive 
in  2015 


https://archive.org/details/developmentalana01arey 


DEVELOPMENTAL 

ANATOMY 

A TEXT-BOOK  AND  LABORAl'ORY  MANUAL  OF  EMBRYOLOGY 


By 

LESLIE  BlEAINERD  AREY 

PROFESSOR  OF  ANATOMY  AT  THE  NORTHWESTERN  UNIVERSITY  MEDICAL  SCHOOL,  CHICAGO 


WITH  419  ILLUSTRATIONS 
MANY  IN  COLOR 


PHILADELPHIA  AND  LONDON 

W.  B.  SAUNDERS  COMPANY 


1924 


(J 


(\)pyriglit.,  JD24,  hy  W.  13.  iSamidcra  Company 


MADE  t N U.  S A 


PREFACE 


This  book  has  been  prepared  for  the  use  of  medical  students 
and  others  whose  interests  center  primarily  on  man  and  mammals.  The 
emphasizing  of  structural  rather  than  functional  aspects  of  Embryology 
is  reflected  in  the  title;  such  presentation  is  consistent  both  with  the 
practical  demands  of  modern  courses  and  with  the  meagre  information 
existant  as  to  the  physiological  factors  in’^developmcnt. 

The  volume  contains  three  sections.  In  the  first  i>art  the  early  stages 
are  treated  comparatively  and  the  fulCcourse  of  prenatal  and  postnatal 
development  is  outlined.  The  second  section  traces  the  origin  and  differ- 
entiation of  the  human  organ-systems,  grouped  according  to  their  germ- 
layer  derivations.  The  third  division  comprises  a laboratory  manual 
for  the  study  of  chick  and  pig  embryos. 

hlany  illustrations  are  from  the  earlier  Prentiss-Arey  text  and 
discontinuous  fragments  of  description  have  likewise  been  retained.  Yet, 
in  plan  and  content  the  work  is  essentially  new.  It  is  hoped  that 
the  developmental  story  has  been  told  in  an  orderly  and  clear,  but  concise 
fashion,  and  that  it  records  accurately  the  present  state  of  the  subject. 

L.  B.  Arey. 

Chicago,  III., 

September,  1924. 


1 


i 


I 


• , t A 


iS' 


'•V 


■ 'V,  ■■*'V; 


CONTENTS 


PART  I.  GENERAL  DEVELOPMENT 

Page 

Introduction i 

General  Features  of  Development 3 

Fundamental  Conceptions 8 

The  Vertebrate  Groups 10 

Titles  for  Collateral  Reading  and  Reference 1 1 

Chapter  I. — The  Germ  Cells  and  Fertilization 12 

The  Germ  Cells 12 

Spermatogenesis,  Oogenesis  and  Maturation 15 

Ovulation  and  Insemination 21 

Fertilization 25 

Heredity  and  Sex 28 

Chapter  II. — Cleavage  and  the  Origin  of  the  Gek.m  Layers 30 

Cleavage 30 

The  formation  of  Ectoderm  and  Entoderm  (Gastrulationj 34 

Origin  of  the  Mesoderm,  Notochord  and  Neural  Tube 36 

Chapter  III. — Implantation  and  Fetal  Membranes 44 

The  Fetal  Membranes  of  Reptiles  and  Birds 44 

The  Fetal  Membranes'of  Mammals 46 

The  Fetal  Membranes  of  Man 49 

Implantation  and  Early  Mucosal  Relations 56 

The  Decidual  Membranes 58 

The  Placenta 62 

Parturition 66 

Chapter  IV. — Age,  Body  Form  and  Growth  Changes 68 

Age,  Size  and  Weight  of  Embryos 68 

An  Outline  of  Prenatal  Development 69 

The  Establishment  of  External  Form 76 

Growth  Changes 81 

PART  II.  ORGANOGENESIS 
Entoderm.al  Derivatives 

Chapter  V. — The  Digestive  System 85 

The  Mouth 87 

The  Pharynx 99 

The  Digestive  Tube 103 

The  Liver 109 

The  Pancreas 1 1 2 

Chapter  VI. — The  Respiratory  System 114 

The  Larynx 115 

The  Trachea 1 16 

The  Lungs 1 1 6 

Me.sodermal  Derivatives 

Chapter  VII. — The  Mesenteries  and  Ccelom 119 

The  Mesenteries 119 

The  Primitive  Mesentery 119 

Differentiation  of  the  Dorsal  Mesentery 120 

Differentiation  of  the  Ventral  Mesentery 125 

The  Ccelom 127 

The  Primitive  Coelom 127 

The  Septum  Transve.rsum 128 


V 


VI 


CONTENTS 


Page 

'J'lic  Pleuro-pcricardial  and  Pleuro-peritoncal  Membranes 129 

The  Pericardium  and  Diaphragm 132 

Chapter  VIII. — T'iie  Urogf.nitae  System 135 

T'hc  Urinary  Organs 135 

d'lie  Pronephros 135 

The  Mesonephros 137 

The  Metanephros 139 

Differentia tion  of  the  Cloaca 145 

The  Genital  Organs 149 

Indifferent  Stage 149 

Internal  Sexual  Transformations 153 

The  External  Genitalia 163 

Homologies  of  Internal  and  External  Genitalia 166 

Chapter  IX.— The  Vascular  System 169 

Origin  of  the  Blood  Vessels  and  Blood  Cells 169 

Hemopoiesis 169 

Development  of  the  Heart 173 

'I'he  Primitive  Vascular  System 185 

I levelopment  of  the  Arteries 189 

Development  of  the  Veins 194 

l''etal  Circulation  and  the  Changes  at  Birth 201 

'I'he  Lymphatic  System 203 

Ohapter  X. — The  Skeletai,  System 206 

Histogenesis  of  the  Supporting  'Fissiu's 206 

Connective  Tissue 206 

Cartilage 208 

Bone 208 

Morphogenesis  of  the  Skeleton 212 

'Fhe  Axial  Skeleton 221 

'Fhe  Appendicular  Skeleton 221 

Chapter  XI. — The  Muscular  System 223 

'Fhe  Histogenesis  of  Muscle 223 

Morphogenesis  of  the  Muscles 224 

Ectodermal  Derivatives 

Chapter  XII. — The  Integumentary  Systfai 230 

'Fhe  vSkin 230 

'Fhe  Nails 231 

'Fhe  Hair 232 

vSebaceous  Glands 234 

Sweat  Glands 234 

Mammary  Glands 235 

Chapter  XIII. — The  Central  Nervous  System 237 

Histogenesis  of  the  Nervous  Tissues 237 

Morphogenesis  of  the  Central  Nervous  System 245 

'Fhe  Spinal  Cord 246 

'Fhe  Brain 251 

Chapter  XIV. — The  Peripheral  Nervous  System 274 

'The  Spinal  Nerves 275 

'Fhe  Cranial  Nerves 278 

The  Sympathetic  Nervous  System 287 

'Fhe  Chromaffin  Bodies  and  Suprarenal  Gland 289 

Chapter  XV. — The  Sense  Organs 292 

General  Sensory  Organs 292 

'Fhe  Gustatory  Organ 292 

'Fhe’ Nose .' 293 

'Fhe  liyc 297 

'Fhe  Ear 3t>5 

PAR'r  III.  A LABORATORY  MANUAL  OF  EAIBRYOLOGY 

Chapter  XVI. — 'Fhe  Study  of  Chick  Embryos 313 

'Fhe  Unincubated  Ovum  and  Embryos  of  the  First  Day 31  3 

Embryo  of  Five  Segments  (Twenty-Three  Hours) 318 

mbryo  of  Seven  Segments  (Twenty-five  Hours) 319 


CONTENTS  Vll 

Page 

Embryo  of  Seventeen  Segments  (Thirty-eight  Hours) 326 

Embryo  of  Twenty-seven  Segments  (Two  Days) 336 

Embryos  of  Three  to  Four  Days 349 

Chapter  XVII. — The  Study  of  Pig  Embryos 353 

The  Anatomy  of  a Six  Mm.  Pig  Embryo 334 

The  iVnatomy  of  Ten  to  Twelve  Mm,  Pig  Embryos 37g 

The  Anatomy  of  an  Eighteen  Mm.  Pig  Embryo 400 

The  Anatomy  of  a Thirty-five  Mm.  Pig  Embryo 402 

Methods  for  the  Dissection  of  Pig  Embryos 405 


Index 409 


'j 


A 


DEVELOPMENTAL 


ANATOMY 


PART  I.  GENERAL  DEVELOPMENT 


INTRODUCTION 

The  Scope  of  Embryology. — Developmental  anatomy,  or  embryol- 
ogy, traces  the  formative  history  of  the  individual  from  the  origin  of  the 
germ  cells  to  the  adult  condition.  Although  the  most  striking  changes 
in  human  development  occur  while  the  young  (called  an  embryo  or  fetus) 
is  still  inside  its  mother’s  womb,  yet  development  by  no  means  ceases  at 
birth.  Birth  is  a mere  incident  which  occurs  when  the  new  individual  is 
sufficiently  advanced  to  allow  its  transference  from  a protected  riterine 
environment  to  one  in  the  external  world.  Some  vertebrates,  like  fishes 
and  amphibia,  are  capable  of  an  active  and  independent  existence  at  very 
immature  stages;  these  free-living  larvee,  as  they  are  termed,  then  gradu- 
ally progress  to  adults.  The  human  newborn,  although  far  more  complete 
anatomically,  is  still  utterly  dependent  for  food  and  care:  many  years  of 
infancy  and  childhood  must  elapse  before  it  becomes  self-maintaining  in 
human  society.  During  all  this  period,  postnatal  development  continues. 
Birth,  itself,  initiates  anatomical  changes  of  profound  influence  on  the 
body.  Throughout  the  entire  growth  period,  with  its  uneven  but 
steadily  slowing  growth  rate,  come  the  completion  of  some  organs  and  a 
gradual  remoulding  of  the  shape  of  the  body  and  its  parts.  Only  at  the 
age  of  twenty-five  are  these  progressive  changes  complete. 

All  vertebrate,  or  backboned,  animals  are  organized  upon  a common 
anatomical  plan,  and  even  many  of  their  structural  details  are  comparable, 
though  superficially  disguised.  vSimilarly,  their  fundamental  mode  of 
development  is  essentially  identical.  The  minor  variations  that  do  occur 
are  caused  by  such  secondary  modifying  factors  as  the  crowding  yolk- 
content  of  the  egg  or  adaptations  to  development  inside  or  outside  the 
mother’s  body.  While  the  comparative  viewpoint  is  indispensable  for 
gaining  a broad  understanding  of  embryology,  it  has  been  of  especial 
importance  in  supplying  missing  parts  of  the  human  developmental  story 
and  in  interpreting  many  perplexing  conditions.  For,  the  earliest  human 
embryos  known  are  about  two  weeks  old  and  have  the  three  primary 
germ  layers  already  formed.  Even  invertebrate  material  is  highly  useful 


INTRODUCTION 


for  demonstrating  such  early  stages  as  maturation,  fertilization,  cleavage, 
and  the  formation  of  blastula  and  gastrula. 

The  Value  of  Embryology. — A general  conception  of  how  man  and 
other  animals  develop  from  a single  cell  by  orderly  and  logical  processes 
should  share  in  the  cultural  background  of  every  educated  mind.  To  the 
medical  student,  embryology  is  of  primary  importance  because  it  affords 
a comprehensive  understanding  of  the  intricacies  and  variations  of  human 
anatomy,  and  thus  is  essential  to  sound  surgical  training.  It  also  explains 
many  anomalies  and  'monstrous’  conditions,  and  the  origin  of  certain 
tumors  and  other  pathological  changes  in  the  tissues.  Obstetrics  is 
essentially  applied  embryology.  From  the  theoretical  side,  it  is  the  key 
with  which  we  may  unlock  the  secrets  of  heredity,  the  determination  of 
sex,  and,  in  part,  of  organic  evolution. 

Histoiical.  - The  science  of  modern  embryology  is  comparatively  new, 
originating  with  the  use  of  the  compound  microscope  and  advancing 
with  the  improvement  of  microscopical  technique.  Aristotle  (384-322 
B.  c.),  however,  centuries  before  the  introduction  of  magnifying  lenses  had 
followed  the  general  development  of  the  chick,  day  by  day.  The  popular 
belief  that  slime  and  decaying  matter  is  capable  of  giving 
rise  to  living  animals,  as  also  asserted  by  Aristotle,  was 
disproved  by  Redi  (1668). 

A few  years  after  Harvey  and  Malpighi  had  published 
their  fundamental  studies  on  the  chick  embryo,  Leeuwenhoek 
reported  the  discovery  of  the  human  .spermatozoon  by  Ham 
in  1677-  At  this  period,  it  was  believed  either  that  fully 
formed  animals  existed  in  miniature  in  the  egg,  needing  only 
the  stimulus  of  the  spermatozoon  to  initiate  development, 
or  that  similarly  preformed  bodies,  male  and  female,  con- 
stituted the  spermatozoa  and  that  these  merely  enlarged 
within  the  ovum.  According  to  this  doctrine  of  preformation, 
all  future  generations  were  likewise  encased,  one  inside  the 
sex  cells  of  the  other,  and  serious  computations  were  made 
as  to  the  probable  number  of  progeny  (200  millon)  thus 
present  in  the  ovary  of  Mother  Eve,  at  the  exhaustion  of  which  the  human 
race  would  end!  Dalenpatius  ( 1699)  and  others  even  believed  they  had 
observed  a minute  human  form  in  the  spermatozoon  (Fig.  i). 

The  preformation  theory  was  strongly  combated  by  Wolff  (1759), 
who  saw  that  the  organs  of  the  early  chick  embryo  were  differentiated 
gradually  from  unspecialized  living  substance.  This  theory,  known  as 
epigenesis,  was  proved  correct  wEen  von  Baer  discQvered  the  mammalian 
ovurn  in  1827,  and  later  demonstrated  the  germ-layer  composition  of 
all  embryos. 


Fig.  I. — Human 
sperm  cell  con- 
taining a minia- 
ture organism,  ac- 
cording to  Hart- 
soeker  (1694). 


GENERAL  FEATURES  OF  DEVELOPMENT 


3 


About  twenty  years  after  Schleiden  and  Schwann  (1839)  had  shown 
the  cell  to  be  the  structural  unit  of  the  organism,  the  ovum  and  spermato- 
zoon were  recognized  as  true  cells.  O.  Hertwig,  in  was  the  first  to 

observe  and  appreciate  the  events  of  fertilization.  Henceforth,  all  multi- 
cellular organisms  were  believed  to  develop  each  from  a single  fertilized 
ovum.  This  conception  is  expressed  in  the  famous  aphorism:  ‘ornne_ 
vivum  ex  ovo.  ’ 

Modern  embryology,  as  an  organized  and  definite  science,  began  with 
Balfour  (1874),  who  reviewed,  digested,  and  made  accessible  the  earlier 
scattered  facts.  Throughout  this  period,  the  experimental  method  of 
investigation  has  been  used  increasingly;  without  it  many  structural  and 
physiological  aspects  of  development  would  remain  unsolved. 

GENERAL  FEATURES  OF  DEVELOPMENT 

A multicellular  embryo  results  from  the  division  of  the  fertilized  ovum 
to  form  daughter  cells.  These  are  at  first  quite  similar  in  structure,  and, 
if  separated,  in  some  animals  each  may  become  a complete  embryo 
(sea  urchin;  certain  vertebrates).  In  general,  the  development  of  an 
embryo  depends:  (i)  upon  the  multiplication  of  its  cells  by  division;  (2) 
upon  the  growth  in  size  of  the  individual  cells;  (3)  upon  changes  in  their 
form  and  structure. 

Cell  Division. — All  cells  arise  from  pre-existing  cells  by  division. 
There  are  two  methods  of  cell  division — amitosis  and  mitosis. 

AMiTOSis. — Cells  may  divide  directly  by  the  simple  fission  of  their 
nuclei  and  cytoplasm.  This  rather  infrequent  process  is  called  amitosis. 
Amitosis  is  said  by  many  to  occur  only  in  specialized  or  moribund  cells. 
It  is  the  type  of  cell  division  demonstrable  in  the  epithelium  of  the 
bladder. 

MITOSIS. — In  the  reproduction  of  typically  active  somatic  cells  and  in 
all  germ  cells,  complicated  changes  take  place  in  the  nucleus.  These 
changes  give  rise  to  thread-like  structures,  hence  the  process  is  termed 
mitosis  (thread)  in  distinction  to  amitosis  (no  threadk  Mitosis  is  divided 
for  convenience  into  four  phases  (Fig.  2) : 

Prophase. — i.  The  centrosome  divides  and  the  two  minute  bodies 
resulting  from  the  division  move  apart,  ultimately  occup^dng  positions  at 
opposite  poles  of  the  nucleus  (I-III). 

2.  Astral  rays  appear  in  the  cytoplasm  about  each  centriole.  They 
radiate  from  it,  and  the  threads  of  the  central  or  achromatic  spindle  are 
formed  between  the  two  asters,  thus  constituting  the  amphiastcr  (II). 

3.  The  nuclear  membrane  and  nucleolus  disappear,  the  karyoplasm 
and  cytoplasm  becoming  confluent. 


4 


INTROUUCTI(.)N 


4.  During  the  above  changes  the  chromatic  network  of  the  resting 
nucleus  resolves  itself  into  a skein,  or  spireme,  which  soon  shortens  and 
l;)reaks  up  into  distinct,  heavily-staining  bodies,  the  chromosomes  (II, 
III).  x\  definite  number  of  chromosomes  is  always  found  in  the  cells  of 


Fig.  2. — Diagrams  of  the  phases  of  mitosis  (Schafer). 


a given  species,  fi'he  chromosomes  may  be  block-shaped,  rod- shaped, 
or  bent  in  the  form  of  a U or  V. 

5.  The  chromosomes  arrange  themselves  in  the  equatorial  plane  of 
the  central  spindle  (IV).  If  U-  or  V-shaped,  the  angle  of  each  is  directed 
toward  a common  center.  The  amphiaster  and  the  chromosomes  together 
constitute  a mitotic  figure,  and  at  the  end  of  the  prophase  this  is  called  a 
monaster. 

Metaphasc. — The  longitudinal  splitting  of  the  chromosomes  into 
exactly  similar  halves  constitutes  the  metaphasc  (IV).  The  aim  of  mitosis 


GENERAL  FEATURES  OF  DEVELOPMENT 


5 


is  thus  accomplished,  an  accurate  division  of  the  chromatin  between  the 
nuclei  of  the  daughter  cells. 

Anaphase. — The  two  groups  of  daughter  chromosomes  separate  and 
move  up  along  the  central  spindle  fibers,  each  toward  one  of  the  two 
asters.  Hence  this  is  called  the  diaster  stage  (V,  VI).  Each  centriole 
may  divide  in  preparation  for  the  next  divi.sion  of  the  daughter  cells. 

Telophase. — i.  The  daughter  chromosomes  resolve  themselves  into 
a reticulum  and  daughter  nuclei  are  formed  (VII,  VIII). 

2.  The  cytoplasm  divides  in  a plane  perpendicular  to  the  axis  of  the 
mitotic  spindle  (VIII).  Two  complete  daughter  cells  have  thus  arisen 
from  the  mother  cell. 

The  number  of  chromosomes  is  constant  in  the  cells  of  a given  species.  The  smallest 
assortment,  two,  occurs  in  Ascaris  megalocephala  univaleiis,  a round  worm  parasitic  in  the 
intestine  of  the  horse.  The  largest  number  known  is  found  in  the  brine  shrimp,  Artemia,  where 
1 68  have  been  counted.  The  chromosome  enumeration  for  the  human  cell  has  been  variously 
stated  but  the  results  of  Winiwarter  (1912),  Grosser  (1921 ),  and  Painter  (1923)  now  agree  on  a 
relatively  high  number,  which  Painter  establishes  as  48  for  whites  and  negroes  of  both  sexes. 

The  Germ  Layers. — The  first  changes  in  the  form  and  arrangement  of 
the  cells  establish  three  definite  plates,  the  primary  germ  layers,  which  are 


Fig.  3.  — Alesenchyme  from  a chick  embryo  (Prentiss).  X 495. 

termed  from  their  positions  the  ectoderm  (outer  skin),  mesoderm  (middle 
skin)  and  entoderm  (inner  skin)  (Fig.  4).  Since  the  ectoderm  covers  the 
body,  it  is  primarily  protective  in  function,  but  it  also  gives  origin  to  the 
nervous  system,  through  which  sensations  are  received  from  the  outer 
world.  The  entoderm,  on  the  other  hand,  lines  the  digestive  canal  and  is 
from  the  first  nutritive.  The  mesoderm,  lying  between  the  other  two 
layers,  naturally  performs  the  functions  of  circulation,  of  muscular  move- 
ment, and  of  excretion;  it  also  gives  rise  to  the  skeletal  structures  which 
support  the  body.  While  all  three  germ  layers  form  definite  sheets  of 
cells  known  as  epithelia,  the  mesoderm  takes  also  the  form  of  a diffuse 
meshwork  of  cells,  the  mesenchyme  (Fig.  3). 


6 


IXTRODUCTION 


The  cells  of  these  layers  are  modified  in  turn  to  form  tissues,  such  as 
muscle  and  nerve,  of  which  the  various  organs  are  composed.  The  organs, 
associated  as  organ  systenis,  constitute  the  organism,  or  body,  that  of 
adult  man  containing  2 5 million  million  red  blood  cells  alone.  In  every 
organ,  one  tissue,  like  the  epithelial  lining  of  the  stomach,  is  pre- 
dominately important;  the  others  are  accessory. 

Histogenesis.--  -The  cells  of  the  germ  layers  are  at  first  alike  in  struc- 
ture. Thus,  the  evagination  which  forms  the  primordial  arm  is  composed 
of  a single  layer  of  similar  ectodermal  cells,  surrounding  a central  mass  of 
diffuse  mesenchyme  (Fig.  406).  Gradually  the  ectodermal  cells  multiply, 
change  their  form  and  structure,  and  give  rise  to  the  layers  of  the  epi- 
dermis. By  more  profound  structural  changes  the  mesenchymal  cells 
ahso  are  transformed  into  the  elements  of  connective  tissue,  tendon, 
cartilage,  bone,  and  muscle — aggregations  of  modified  cells  which  are 
termed  tissues.  The  development  of  modified  tissue  cells  from  the 
undifferentiated  cells  of  the  germ  layers  is  known  as  histogenesis. 

During  histogenesis,  the  structure  and  form  of  each  tissue  cell  are 
adapted  to  the  ])erformance  of  some  special  function  or  functions.  Cells 
which  have  once  taken  on  the  structure  and  functions  of  a given  tissue 
cannot  give  rise  to  cells  of  any  other  type.  In  tissues  like  the  epidermis, 
certain  cells  retain  their  ])rimitive  embryonic  characters  throughout  life, 
and,  by  continued  cell  division  produce  new  layers  of  cells  which  are 
later  specialized.  In  other  tissues  all  of  the  cells  are  differentiated  into 
the  adult  type,  after  which  no  new  cells  are  formed:  this  takes  place  in 
the  nervous  elements  of  the  central  nervous  system.  Contrariwise,  most 
tissue  cells  are  undergoing  retrogressive  changes  throughout  life.  In 
this  way,  the  cells  of  certain  organs  like  the  thymus  gland  and  meso- 
nephros degenerate  and  largely  disappear.  The  cells  of  the  hairs  and  the 
surface  layer  of  the  epidermis  become  cornified  and  eventually  are  shed. 
Thus,  normally,  many  tissue  cells  are  continually  being  destroyed  and 
replaced  by  new  cells. 

This  series  of  changes — an  embryonic  (undifferentiated)  stage; 
progressive  functional  s])ecialization ; gradual  degeneration;  death  and 
removal — which  tissue  cells  experience  is  designated  by  the  term 
cytomorphosis. 

Derivatives  of  the  Germ  Layers. — The  tissues  of  the  adult  are  derived 
from  the  primary  germ  layers  as  follows: 


Ectoderm 


Mesoderm 


Entoderm 
Epithelium  of: 

I.  Pharynx  and  derivatives. 
Auditory  tube. 

Tonsils. 

Thymus. 

Thyroid. 


I.  Epidermis  and  derivatives. 
Hair;  nails;  glands. 

Lens  of  eye. 


A.  Mesothelium. 

1.  Pericardium. 

2.  Pleura. 

3.  Peritoneum. 

4.  LTrogenital  epithelia. 

5.  Striated  muscle. 


2.  Epithelium  of: 

Organs  of  special  sense. 
Cornea. 


GENERAL  EEATURES  OF  DEVELOPMENT 


/ 


Ectoderm 


Mesoderm 


Entoderm 


Mouth;  enamel  organ. 
Oral  glands;  hypophysis. 
Anus. 

Amnion;  chorion. 


B.  Mesenchyme. 


Parathyroid. 

2.  Respiratory  tract. 


1 . Smooth  muscle. 

2.  Notochord. 

3.  Connective  tissue; 


Lungs. 

3.  Digestive  tract. 


Larynx;  trachea. 


3.  Nervous  tissue. 
Neuroglia. 
Chromaffin  tissue. 


cartilage;  bone. 

4.  Blood;  bone  marrow. 

5.  Endothelium  of  blood 


Yolk  sac;  allantois. 

4.  Bladder  (except  trigone). 

5.  LTrethra  (except  prostatic). 

6.  Prostate. 


Liver;  pancreas. 


4.  Smooth  muscle  of; 
Iris. 

Sweat  glands. 


vessels  and  lymphatics. 

6.  Lymphoid  organs. 

7.  Suprarenal  cortex. 


Primitive  Segments — Metamerism. — A prominent  feature  of  verte- 
brate embryos  are  the  primitive  segments,  or  metameres  (Fig.  59).  These 
segments  are  homologous  to  the  serial  divisions  of  an  adult  earth-worm’s 
body,  divisions  which,  in  the  earth  worm,  are  identical  in  structure,  each 
containing  a ganglion  of  the  nerve  cord,  a muscle  segment,  or  myotome, 
and  pairs  of  blood  vessels  and  nerves.  In  vertebrate  embryos,  the  block- 


Coelom 

Fig.  4. — Diagrammatic  transverse  section  of  a vertebrate  embryo  (Minot-Prentiss). 

like  primitive  segments  lie  next  the  neural  tube  and  are  known  as  meso- 
dermal segments,  or  somites  (Fig.  4).  Each  pair  gives  rise  to  a vertebra,  to 
two  myotomes,  or  muscle  segments,  and  to  paired  vessels;  each  set  of 
mesodermal  segments  is  supplied  by  a pair  of  spinal  nerves:  consequently, 
the  adult  vertebrate  body  is  segmented  like  that  of  the  earth  worm. 
As  a worm  grows  by  the  formation  of  new  segments  at  its  tail-end,  so  the 
metameres  of  the  vertebrate  embryo  begin  to  form  in  the  head  and  are 
added  tailward.  There  is  this  difference  between  the  segments  of  the 
worm  and  the  vertebrate  embryo;  the  segmentation  of  the  worm  is 
complete,  while  that  of  the  vertebrate  is  incomplete  ventrally. 


Notochord 


Neural  tithe 


SplancliHO- 
pleure  \ 


8 


INTRODUCTION 


Somatopleure  and  Splanchnopleure.  In  early  embryos  the  meso- 
derm splits  into  two  layers,  the  somatic  (dorsal)  and  splanchnic  (ventral) 
mesoderm  (Fig.  4).  The  ectoderm  and  somatic  mesoderm  constitute 
the  lu)dy  wall,  which  is  termed  the  somatopleure.  In  the  same  way,  the 
entoderm  and  splanchnic  mesoderm  combine  as  the  splanchnopleure; 
it  forms  the  mesenteries  and  the  walls  of  the  gut,  heart,  and  lungs. 

Ccelom.^  -The  space  between  the  somatopleure  and  splanchnopleure 
is  the  ccclom,  or  body  cavity.  At  the  first  splitting  of  the  mesoderm,  iso- 
lated clefts  are  produced.  These  unite  on  each  side  and  eventually 
form  one  cavity — the  coelom.  With  the  extension  of  the  mesoderm,  the 
cot'lom  surrounds  the  heart  and  gut  ventrally  (Fig.  4).  Later,  it  is  sub- 
divided into  the  pericardial  caznty  about  the  heart,  the  pleural  cavity  of 
the  thorax,  and  the  peritoneal  cavity  of  the  abdominal  region.  The 
ci)ithelia  lining  the  several  body  cavities  are  termed  mesothelia. 

The  Nephrotome.  — The  bridge  of  cells  connecting  the  primitive 
segment  with  the  unsegmented  somatic  and  splanchnic  layers  is  the 
nephrotome,  or  intermediate  cell  mass  (Fig.  4).  From  these  will  develop 
the  urogenital  glands  and  ducts. 

D evelopmental  Processes. — The  developing  embryo  exhibits  a 
])rogressively  comjilex  structure,  the  various  steps  in  the  production  of 
which  occur  in  orderly  sequence.  There  may  be  recognized  in  develop- 
ment a number  of  component  mechanical  processes  which  are  used 
repeatedly  by  the  embryo.  The  general  and  fundamental  process  condi- 
tioning ilifferentiation  is  cell  multiplication,  and  the  subsequent  growth  of 
the  daughter  cells.  The  more  important  of  the  specific  developmental 
])rocesses  are  the  following:  ( i)  cell  migration;  (2)  localized  growth,  resulting 
in  eidargements  and  constrictions;  (3)  cell  aggregation,  forming  (a)  cords, 
(b)  sheets,  [c]  masses;  (4)  delamination,  that  is,  the  splitting  of  single  sheets 
into  separate  layers;  (5)  folds,  including  circumscribed  folds  which  produce 
ia)  evaginations,  or  out-pocketings,  (b)  invaginations,  or  in-])Ocketings. 

1 he  production  of  folds,  including  evaginations  and  invaginations,  due 
to  unequal  rapidity  of  growth,  is  the  chief  factor  in  moulding  the  organs  and 
hence  the  general  form  of  the  embryo. 

FUNDAMENTAL  CONCEPTIONS 

The  Anlage.  — This  German  word,  which  lacks  an  entirely  satisfactory 
English  equivalent,  is  a term  applied  to  the  first  discernible  cell,  or  aggre- 
gation of  cells,  which  is  destined  to  form  any  distinct  jiart  or  organ  of  the 
embryo.  In  the  broad  sense,  the  fertilized  ovum  is  the  anlage  of  the  entire 
adult  organism;  furthermore,  in  the  early  cleavage  stages  of  certain 
embryos  it  is  possible  to  recognize  single  cells  or  cell  groups  from  which 
definite  structures  will  indubitably  arise.  The  term  anlage,  however,  is 


FUNDAMENTAL  CONCEPTIONS 


9 


more  commonly  applied  to  the  primordia  that  differentiate  from  the 
various  germ  layers.  Thus  the  epithelial  thickening  over  the  optic 
vesicle  is  the  anlage  of  the  lens. 

The  Law  of  Genetic  Restriction. — As  development  advances,  there  is  a 
constantly  increasing  restriction  in  the  kind  of  differentiation  open  to  the 
various  parts.  Each  emerging  tissue  or  organ  is  more  rigidly  bound  to 
its  particular  type  of  differentiation  than  was  the  generalized  material 
from  which  it  came.  A line  of  specialization,  once  begun,  cannot  be 
abandoned  for  another  type.  The  parent  tissue,  likewise,  is  limited  by 
losing  the  capacity  for  duplicating  anlages  already  formed.  Thus,  the 
primitive  thyroid  can  never  become  anything  but  a thyroid,  whereas  the 
gut  that  formed  it  also  buds  off,  at  other  levels,  the  lungs,  liver,  and 
pancreas.  Yet  if  the  embryonic  thyroid  were  destroyed,  the  pharynx 
would  never  replace  it.  From  mesenchyme  arise  connective  tissue, 
blood  cells,  and  smooth  muscle;  when  once  the  specialization  begins,  there 
can  be  no  retraction  or  transformation  to  another  type. 

Continuity  of  the  Germ  Plasm. — According  to  this  important  con- 
ception of  Weismann,  the  body-protoplasm,  or  soma,  and  the  reproduc- 
tive-protoplasm differ  fundamentally.  The  germinal  material  is  a legacy 
that  has  existed  since  the  beginning  of  life,  from  which  representative 
portions  are  passed  on  intact  from  one  generation  to  the  next.  Around 
this  germ  plasm  there  develops  in  each  successive  generation  a short- 
lived body,  or  soma,  which  serves  as  a vehicle  for  insuring  its  transmission 
and  perpetuation.  The  reason,  therefore,  why  offspring  resembles 
parent  is  because  each  develops  from  portions  of  the  same  stuff. 

The  Law  of  Biogenesis.- — Of  great  theoretical  interest  is  the  fact, 
constantly  observed  in  studying,  embryos,  that  the  individual  in  its 
development  repeats  hastily  and  incompletely  the  evolutionary  history 
of  its  own  species.  This  law  of  recapitulation  was  first  stated  clearly  by 
Muller  in  1863,  and  was  termed  by  Haeckel  the  law  of  biogenesis.  In 
accordance  with  it,  the  fertilized  ovum  is  compared  to  a unicellular 
organism  like  the  Ameba:  the  blastula  is  supposed  to  represent  an  adult 
Volvox  type;  the  gastrula,  a simple  sponge;  the  segmented  embryo,  a 
worm-like  stage ; and  the  embryo  with  gill  slits  may  be  regarded  as  a fish- 
like stage.  Moreover,  the  blood  of  the  human  embryo  in  development 
passes  through  stages  in  which  its  corpuscles  resemble  in  structure  those 
of  the  fish  and  reptile;  the  heart  is  at  first  tubular,  like  that  of  the  fish, 
and  the  arrangement  of  blood  vessels  is  equally  primitive;  the  kidney  of 
the  embryo  is  like  that  of  the  amphibian,  as  are  also  the  genital  ducts. 
Many  other  examples  of  this  law  may  readily  be  observed. 

Some  apparently  useless  structures  appear  during  development, 
perfunctorily  reminiscent  of  ancestral  conditions;  certain  other  parts,  of 


TO 


INTRODUCTION 


use  to  the  embryo  alone,  are  later  replaced  by  better-adapted,  permanent 
organs.  Representatives  of  either  type  may  eventually  disappear  or 
they  may  persist  throughout  life  as  rudimentary  organs;  more  than  a 
hundred  of  the  latter  have  been  listed  for  man.  Still  other  ancestral 
organs  abandon  their  provisional  embryonic  function,  yet  are  retained  in 
the  adult  and  utilized  for  new  purposes. 

THE  VERTEBRATE  GROUPS 

There  are  five  vertebrate  classes,  the  higher  characterized  by  the 
possession  of  an  enveloping  embryonic  membrane,  called  the  amnion,  and 
another  embryonic  appendage,  known  as  the  allantois: 

(R)  Anamniota  (amnion  absent). 

1.  Fishes — lamprey;  sturgeon;  shark;  bony  fishes;  lung  fish. 

2.  Amphibia — ^salamander;  frog;  toad;  etc. 

{B)  Amniota  (amnion  present). 

3.  Reptiles — lizard;  crocodile;  snake;  turtle. 

4.  Birds. 

5.  hlammals.  Characterized  by  hair  and  mammary  glands, 
(a)  Monotremes — duck-bill;  primitive  mammals  that  have  a 

cloaca  and  lay  eggs  with  shells. 

ih)  Marsupials — oppossum;  kangaroo;  etc.  The  young  are 
born  immature  and  are  sheltered  in  an  integumentary  pouch, 
(r)  Placentalia.  All  other  mammals  whose  young  are  nour- 
ished in  the  uterus  by  a placenta. 

Ungulate  series.  Hoofed  mammals  (cattle;  sheep;  pig; 
deer;  horse;  etc.). 

Unguiculate  series.  Clawed  mammals  (mole;  bat;  rat; 
rabbit;  cat;  dog;  etc.).  The  highest  order  is  the  Primates 
(lemur;  monkey;  ape,  man). 

The  Vertebrate  Body  Plan. — All  vertebrate  animals  are  constructed 
in  accordance  with  a common  body  plan.  The  distinctive  characteristics 
of  the  vertebrate  type  include: 

1.  A tubular  central  nervous  system,  dorsally  placed  (Fig.  4). 

2.  A notochord,  between  the  neural  tube  and  gut  (Fig.  4).  This 
cellular  |3rimitive-axis  is  replaced,  wholly  or  in  part,  by  the  vertebral 
column. 

3.  A pharynx,  which  develops  paired  pouches  and  clefts  that  deter- 
mine the  positions  of  important  nerves,  muscles  and  blood  vessels  (Fig. 
91). 

4.  The  position  of  the  mouth.  Unlike  the  condition  in  many  inverte- 
brates, it  is  not  surrounded  by  a circumoral  ring  of  nervous  tissue  which 
connects  a dorsal  ‘brain’  with  a ventral  chain  of  ganglia. 


THE  VERTEBRATE  GROUPS 


II 


5.  The  limbs.  Two  pairs,  with  an  internal  skeleton  (Fig.  227). 

6.  A coelom,  which  is  divided  into  a dorsal,  segmental  part  (cavities 
of  the  somites),  and  a ventral,  unsegmented  part,  partitioned  by  the 
septum  transversum  (diaphragm)  into  thoracic  and  abdominal  portions 

(Fig.  4 • 

TITLES  FOR  COLLATERAL  READING  AND  REFERENCE 

Broman.  Normale  und  abnorme  Entwicklung  des  Menschen. 

Corning.  Entwicklungsgeschichte  des  Menschen. 

Duval.  Atlas  D’Embryologie. 

Hertwig.  Handbuch  der  Entwicklungslehre  der  Wirbeltiere. 

Keibel  and  Mall.  Human  Embryology. 

Kellicott.  A Textbook  of  General  Embryology. 

Kollmann.  Handatlas  der  Entwicklungsgeschichte  des  Menschen. 

Lillie.  The  Development  of  the  Chick. 

Minot.  A Laboratory  Text-book  of  Embryology. 

McMurrich.  The  Development  of  the  Human  Body. 

Patten.  The  Early  Embryology  of  the  Chick. 

Wilson.  The  Cell  in  Development  and  Inheritance. 


CHAPTER  I 


THE  GERM  CELLS  AND  FERTILIZATION 
THE  GERM  CELLS 

All  multicellulai'  animals,  except  a few  invertebrates,  result  from  the 
union  of  two  ripe  sex  cells.  These  are  re])resentative  portions  of  the  germ 
plasm  stored  in  the  male  and  female  sex  glands,  and  are  termed  sperma- 
tozoon and  ovinn  respectively.  In  form  and  function  they  are  quite  unlike, 
for  each  is  adapted  to  a specific  purpose.  It  will  be  simplest  first  to 
describe  these  elements  fully-formed,  and  then  to  show  how  they  develop, 
mature,  meet,  and  unite. 

The  Ovum. — The  female  germ  cell,  or  ovum,  is  a typical  animal 
cell  produced  in  the  ovary.  Although  always  large,  its  exact  size  is 
correlated  with  the  amount  of  stored  food  substance.  The  smallest  eggs 
are  those  of  the  mouse  and  deer  (about  0.07  mm.).  The  largest  have  a 


Fig.  5. — ( )vum  of  monke}^  (Prentiss).  X 430. 

diameter  measurable  in  inches  (birds;  a shark).  Most  ova  are  nearly 
spherical  in  form  and  pos.sess  a nucleus  with  nucleolus,  chromatin  network, 
and  nuclear  membrane  (Figs.  5 and  7).  The  nucleus  is  essential  to  the 
life,  growth,  and  reproduction  of  the  cell.  The  function  of  the  nucleolus 
is  unknown;  the  chromatin  bears  the  hereditary  qualities.  The  cyto- 
plasm is  distinctly  granular  and  contains  more  or  less  numerous  yolk 
granules,  mitochondria,  and  rarely  a minute  centrosome. 

The  yolk,  or  deutoplasm,  containing  a fatty  substance  termed 
lecithin,  furnishes  nutriment  for  the  developing  embryo.  It  is  doubtful 
if  any  ovum  is  totally  devoid  of  yolk,  yet  it  is  useful  as  a basis  for  classi- 

1 2 


THE  GERM  CELLS 


13 


fying  eggs.  Those  ova  which  contain  relatively  little  yolk,  uniformly 
distributed,  are  termed  isolecithal.  Examples  are  found  among  various 


Chalaza 


Yellow  yolk  White  yolk  Blastoderm 


V tileline  membrane  Abmluen 


Inner  shell  membrane 
Air  chamber 
Shell 

Outer  shell  membrane 
Chalaza 


Fig.  6. — Diagrammatic  longitudinal  section  of  a hen’s  egg  (Thomson  in  Heisler). 


Fig.  7. — .4,  Human  ovum,  approaching  maturity,  examined  fresh  in  the  liquor  folliculi 
(Waldeyer).  X 415.  The  zona  pellucida  appears  as  a clear  girdle  surrounded  by  the  cells  of 
the  corona  radiata.  Yolk  granules  in  the  cytoplasm  enclose  the  nucleus  and  nucleolus.  5,  A 
human  spermatozoon  correspondingly  enlarged. 

invertebrates  and  in  all  placental  mammals,  for  such  embryos  either 
attain  an  independent  existence  quickly  or  are  sheltered  and  nourished 


14 


THE  GERM  CELLS  AND  FERTILIZATION 


within  the  uterine  wall  of  the  mother.  If  the  yolk  collects  at  one  end 
(called  the  vcgdal  pole  in  contrast  to  the  more  jmrely  jirotoplasmic  animal 
pole)  the  ova  are  said  to  he  teloleciihal.  Many  invertebrates  and  all 


Pcrforaloriuni 


vertebrates  lower  than  the  Placentalia 
illustrate  this  type.  The  so-called  yolk 
of  the  hen’s  egg  (Fig.  6)  is  the  ovum 
proper  and  its  yellow  color  is  due  to  the 
large  amount  of  lecithin  it  contains. 
Finally,  among  the  arthropods  the  yolk 
is  centrally  located  and  surrounded  by 
a peripheral  shell  of  clear  cytoplasm; 
such  eggs  are  centrolccithal. 

Most  ova  become  enclosed  within 
protective  membranes,  or  envelopes. 
The  vitelline  membrane,  secreted  by  the 
egg  itself,  is  a primary  membrane  (Fig. 
5).  The  follicle  cells  about  the  ovum 
usually  furnish  other  secondary  mem- 
branes, such  as  the  zona  pellucida.  In 
lower  vertebrates  tertiary  membranes 
may  be  added  as  the  egg  passes  through 
the  oviduct  and  uterus ; the  albumen  and 
shell  of  the  hen’s  egg  (Fig.  6)  or  the  jelly 
of  the  frog’s  egg  are  of  this  sort. 

The  Human  Ovum. — This  is  rela- 
tively of  small  size,  measuring  about  0.2 
mm.  in  diameter  (Fig.  7).  It  conforms 
closely  to  the  isolecithal  mammalian 
type,  but  has  fine  yolk  granules  some- 
what condensed  centrally.  There  is 
apparently  a very  delicate  vitelline 
membrane,  and  outside  it  a thick,  radi- 
ally-striate  membrane,  the  zona  pel- 
lucida. The  striate  appearance  is  said 
to  be  due  to  fine  canals  through  which 
nutriment  is  transferred  from  smaller 
follicle  cells  during  the  growth  of  the 
ovum  within  the  ovary. 

The  Spermatozoon. — -In  a few  instances  only,  does  the  mature  male 
element,  or  spermatozoon,  resemble  a typical  cell.  Most  are  slender, 
elongate  structures  which  develop  a flagellum  to  accomplish  the  active 
swimming  that  characterizes  the  cell.  Unlike  the  ovum,  which  is  the 


Fig.  8. — .1,  Diagram  of  a human 
spermatozoon,  surface  view  (Meves).  B, 
Human  spermatozoa,  from  life,  in  edge 
and  surface  view.  X 700. 


THE  GERM  CELLS 


15 


largest  cell  of  an  organism,  the  spermatozoon  is  usually  the  smallest.  The 
extremes  of  size  range  from  0.018  mm.  in  Amphioxus  to  2.0  mm.  in  an 
amphibian.  The  commonest  shape  is  that  of  an  elongate  tadpole,  with 
an  enlarged  head,  short  neck  (and  connecting  piece),  and  thread-like 
tail  (Fig.  8). 

The  Human  Spermatozoon. — The  sperm  of  man  is  of  average  size  (0.055 
mm.)  and  shape  (Fig.  8).  Compared  to  the  ovum  its  volume  is  as  i : 200,000 
(Fig.  7).  The /zcah  is  about  0.005  mm.  in  length.  It  appears  oval  in  surface 
view,  pear-shaped  in  profile.  When  stained,  the  anterior  two-thirds  of 
the^head  may  be  seen  to  constitute  a cap,  and  the  sharp  border  of  this  cap 
is  the  so-called  perforatorium.  The  head  contains  the  nuclear  elements 
of  the  sperm  cell.  The  disc-shaped  neck  includes  the  anterior  centrosonial 
body.  The  tail  begins  with  the  posterior  centrosonial  body  and  is  divided 
into  a short  connecting  piece,  a chief  piece,  or  flagellum,  which  forms  about 
four-fifths  of  the  length  of  the  sperm  cell,  and  a short  end  piece,  or  terminal 
filament.  The  connecting  piece  is  marked  off  from  the  chief  piece  by  the 
annulus.  The  connecting  piece  is  traversed  by  the  axial  filament  {fdvLm 
principale),  and  is  surrounded:  (i)  by  the  sheath  common  to  it  and  to  the 
flagellum;  (2)  by  a sheath  containing  a spiral  filament;  and  (3)  by  a mito- 
chondrial sheath.  The  chief  piece  is  composed  of  the  axial  filament,  sur- 
rounded by  a cytoplasmic  sheath,  while  the  end  piece  comprises  the  naked 
continuation  of  the  axial  filament. 

Atypical  spermatozoa  occur  in  some  individuals.  These  include 
giant  and  dwarf  forms,  and  elements  with  multiple  heads  or  tails. 

Comparison  of  the  Ovum  and  Spermatozoon. — The  dissimilar  male  and 
female  sexual  cells  are  admirably  adapted  to  their  respective  functions, 
and  illustrate  nicely  the  modifications  that  accompany  a physiological 
division  of  labor.  Each  has  the  same  amount  of  chromatin,  although  in 
the  sperm  it  is  more  compactly  stored.  The  cells  thus  participate  equally 
in  heredity.  The  egg  contains  an  abundance  of  cytoplasm  (but  nO' 
centrosome),  and  often  a still  greater  supply  of  stored  food.  As  a result, 
it  is  large  and  passive,  yet  closely  approximates  the  typical  cell.  On  the 
contrary,  the  sperm  is  small,  and  at  casual  inspection  bears  slight  resem- 
blance to  an  ordinary  cell.  Its  cytoplasm  is  reduced  to  a bare  minimum 
and  contains  no  deutoplasm.  Structurally,  all  is  subordinated  to  a motile 
existence.  Correlated  with  small  size  is  an  extraordinary  increase  in 
numbers,  for  the  greater  the  total  liberated  the  more  surely  will  the  ovum 
be  found.  Hence,  apart  from  its  role  in  heredity,  the  chief  function  of  the 
spermatozoon  is  to  seek  the  ovum  and  activate  it  to  divide. 

SPERMATOGENESIS,  OOGENESIS  AND  MATURATION 

In  becoming  specialized  germ  cells,  the  ovum  and  spermatozoon  pass 
through  parallel  stages.  The  general  process  of  sperm  formation  is 


THE  GERM  CELLS  AND  FERTILIZATION 


1 6 


designated  spermatogenesis;  that  of  egg  formation,  oogenesis.  An  essential 
feature  of  lioth  is  a component  process,  termed  maturation,  which  is 
important  for  the  following  reason.  Since  reproduction  in  vertebrates 
depends  upon  the  union  of  male  and  female  germ  cells,  it  is  manifest  that 
without  special  provision  this  union  would  necessarily  double  the  number  of 
chromosomes  at  each  generation.  Such  progressive  increase  is  prevented 
by  the  events  of  maturation.  This  may  be  defined  as  a form  of  cell  divi- 
sion during  which  the  number  of  chromosomes  in  the  germ  cells  is  reduced 
to  one-half  the  number  characteristic  for  the  species.  Its  significance  in 
the  mechanism  of  inheritance  is  discussed  on  p.  28. 

Spermatogenesis. — The  spermatozoa  originate  in  the  epithelial  lining 
of  the  testis  tubules.  Two  types  of  cells  are  recognizable : the  sustentacular 
cells  (of  Sertoli),  and  the  male  germ  cells  (Fig.  9).  All  the  latter  are 


Sp'z.II  [telophase) 
Sp'c.II  [mdaphase) 


Sp'c.I  [pro phase)  ___ 
Sustentacular  cell 


Conneclivc-tissuc 

wall 


Sp'c.II  [telophase) 

Primary  spermatocyte 
A ccessory  chromo- 
some [?) 

Sp'c.  I [meta phase) 
Sp'c.  I [prophase) 

Spermatogonium 


Sp'g.  [anaphase) 

Fig.  9. — Stages  in  the  spermatogenesis  of  man  arranged  in  a composite  to  represent  a portion 
of  a seminiferous  tubule  sectioned  transversely.  X 900. 


Spermatid 


S permatozoa 


descendants  of  primordial  germ  cells,  which,  by  division,  first  form  spermato- 
gonia. These  in  turn  |3roliferate  and  produce  numerous  generations  of 
like  cells.  Ultimately  the  spermatogonia  enter  a growth  period,  at  the  end 
of  which  they  are  termed  primary  spermatocytes.  Each  contains  the  full 
number  of  chromosomes  typical  for  the  male  of  the  species.  Next  ensues 
the  process  of  maturation.  This  comprises  two  cell  divisions,  each  primary 
spermatocyte  producing  two  secondary  spermatocytes,  and  these  in  turn 
four  cells  known  as  spermatids.  During  these  cell  divisions  the  number  of 
chromosomes  is  reduced  to  half  the  original  number  in  the  spermatogonia. 


SPERMATOGENESIS 


17 


The  spermatids  now  attach  to  Sertoli  cells,  from  which  they  appear  to 
receive  nutriment,  and  become  transformed  into  mature  spermatozoa 
(Fig.  10).  The  nucleus  forms  almost  all  the  head;  the  centrosome  divides, 
the  resulting  particles  passing  to  the  extremities  of  the  neck.  The 
posterior  centrosome  differentiates  the  annulus  and  is  prolonged  to 
become  the  axial  filament.  The  cytoplasm  forms  the  sheaths  of  the 
neck  and  tail,  whereas  the  spiral  filament  of  the  connecting  piece  is  derived 
from  cytoplasmic  mitochondria.  When  the  transformation  is  complete, 
the  spermatozoa  detach  from  the  sustentacular  cells  and  are  set  free  in  the 
lumen  of  the  seminiferous  tubule. 

Maturation  in  Ascaris. — The  way  the  number  of  chromosomes  is  reduc- 
ed may  be  seen  in  the  spermatogenesis  of  Ascaris  (Fig.  1 1).  Four  chromo- 


Fig.  10. — Diagrams  of  the  development  of  spermatozoa  (Meves  in  Lewis  and  Stohr).  a.c.. 
Anterior  centrosome;  a.f.,  axial  filament;  c.p.,  connecting  piece;  ch.p.,  chief  piece;  g.c.,  cap;  «., 
nucleus;  nk.,  neck;  p.,  cytoplasm;  p.c.,  posterior  centrosome. 

somes  are  typical  for  Ascaris  megalocephala  hivalens,  and  each  sperma- 
togonical  cell  contains  this  number.  In  the  early  prophase  of  the  primary 
spermatocyte  there  appears  a spireme  thread  consisting  of  four  parallel 
rows  of  granules  (B).  This  thread  breaks  in  two  and  forms  two  quadruple 
structures,  known  as  tetrads  (D-F) ; each  is  equivalent  to  two  original  chro- 
mosomes, paired  side  by  side  and  split  lengthwdse  to  make  a bundle  of  four. 
At  the  metaphase  {G},  a tetrad  divides  into  its  two  original  chromosomes 
which  already  show  evidence  of  longitudinal  fission  and  are  termed  dyads. 
One  pair  of  dyads  goes  to  each  of  the  daughter  cells,  or  secondary  sperma- 
tocytes (G-I).  Without  the  formation  of  a nuclear  membrane,  the  second 


i8 


THE  GERM  CELLS  AND  FERTILIZATION 


maturation  spindle  appears  at  once,  the  two  dyads  split  into  four  monads, 
and  each  daughter  spermatid  receives  two  single  chromosomes  (monads), 
or  one-half  the  number  characteristic  for  the  species.  The  tetrad,  there- 
fore, represents  a precocious  division  of  the  chromosomes  in  preparation  for 


I 


II 


Fir..  II. — Reduction  of  chromosomes  in  the  spermatogenesis  of  Ascaris  megalocephala 
bivalens  (Brauer  in  Wilsonj.  X about  i loo.  A-G,  Successive  stages  in  the  division  of  the  pri- 
mary spermatocyte.  The  original  reticulum  undergoes  a very  early  division  of  the  chromatin 
granules  which  then  form  a quadruply  split  spireme  (B,  in  profile).  This  becomes  shorter  (C, 
in  profile),  and  then  breaks  in  two  to  form  two  tetrads  (D,  in  profile),  (E,  on  end).  F,  G,  H,  first 
division  to  form  two  secondary  spermatocytes,  each  receiving  two  dyads.  I,  Secondary  sperma- 
tocyte. J,  K,  The  same  dividing.  L,  Two  resulting  spermatids,  each  containing  two  monads 
or  chromosomes. 


two  rapidly  succeeding  cell  divisions  which  occur  without  the  intervention 
of  the  customary  resting  periods.  The  easily  understood  tetrads  are  not 
formed  in  most  animals,  although  the  outcome  of  maturation  is  identical  in 
either  case.  A diagram  of  maturation  is  shown  in  Fig.  12.  The  first 
maturation  division  in  Ascaris  is  probably  reductional,  each  daughter 


OOGENESIS 


19 


nucleus  receiving  two  complete  chromosomes  of  the  original  four,  whereas 
in  the  second  maturation  division,  as  in  ordinary  mitosis,  each  daughter 
nucleus  receives  a half  of  each  of  the  two  chromosomes,  these  being 
split  lengthwise.  The  latter  division  is  eqiiaiional  and  the  daughter  nuceli 
receive  chromosomes  bearing  similar  hereditary  qualities. 

Some  animals  reverse  the  sequence  of  events,  reduction  occurring 
at  the  second  maturation  division. 

Maturation  in  Man. — All  spermatogonia,  like  the  somatic  cells,  con- 
tain 48  chromosomes.  The  primary  spermatocytes  form  tetrads  and  their 
division  separates  the  mated  chromosomal  pairs  into  24  single  chromo- 


A 

Spermatogonium 


B 

Oogonium 


ProUfcralwn 

period 


Growth 

period 


Maturation 

period 


Transforma- 
tion period  of 
spermatozoa 


Frc.  12.— Diagrams  of  maturation  in  spermatogenesis  and  oogenesis  (Boveri). 


somes  of  the  secondary  spermatocyte.  Hence,  this  mitosis  is  reductional. 
The  secondary  spermatocytes  then  divide  equationally  into  spermatids, 
each  of  which  also  contains  24  single  chromosomes.  Transformation  into 
spermatozoa  ensues  (Figs.  9 and  10).  Those  details  of  maturation  which 
pertain  to  sex  determination  are  explained  on  p.  29. 

Oogeneiss. — The  ova,  like  the  male  elements,  arise  from  the  multipli- 
cation of  primordial  germ  cells  in  the  ovary  (cf.  p.  156).  At  birth,  or 
shortly  after,  human  ova  cease  forming.  The  number  at  this  time  in  both 
ovaries  has  been  placed  between  100,000  and  800,000.  Cellular  degenera- 
tion reduces  this  supply  until,  at  18  years,  the  total  is  from  35,000  to  70,000 
and  several  years  after  the  menopause  no  more  are  to  be  found. 


20 


THE  GERM  CELLS  AND  FERTILIZATION 


Late  in  fetal  life,  indifferent  cells,  by  surrounding  the  young  ova 
ioogonia)  of  the  cortex,  produce  primordial  follicles  (Fig.  13  A).  Some 
begin  growth  at  once,  others  are  quiescent  until  childhood  or  adult  life  is 
attained.  During  the  slow  growth  period,  the  small,  nutritive  follicle 
cells  increase  in  number  and  the  oogonium  gains  greatly  in  size.  When  the 
follicle  cells  are  several  layers  deep,  a cavity  appears  between  them. 
This  enlarges,  and  there  re.sults  a sac,  the  vesicular,  or  Graafian  follicle, 
filled  with  fluid,  the  liquor  folliculli  (Fig.  13  5).  As  growth  continues,  the 
oogonium  becomes  located  more  and  more  eccentrically  until  it  lies  at  one 
side  of  the  follicle,  buried  in  a mound  of  follicular  cells  termed  the  cumulus 
odphorus  (egg-bearing  hillock)  (Fig.  14).  Around  the  stratified  follicle 


Fig.  13. — .4 , Two  primordial  human  follicles  and  one  early  in  growth  (De Lee).  X200.  B, 
Section  of  a human  ovarian  cortex  with  ten  primordial  follicles  and  one  young  Graafian  follicle 
(Piersol).  X 90. 

cells,  now  designated  the  stratum  granulosum,  there  is  differentiated  from 
the  stroma  of  the  ovary  the  theca  folliculi.  This  is  composed  of  an  inner, 
vascular  tunica  interna,  and  an  outer,  fibrous  and  muscular  tunica  externa. 

At  the  end  of  the  growth  period,  the  follicle  has  enlarged  from  a 
structure  0.04  to  0.06  mm.  in  diameter  to  one  5 to  12  mm.  (Fig.  16  A); 
similarly,  the  primordial  ovum  measured  0.04  to  0.05  mm.  whereas  it 
now  has  a diameter  of  about  0.2  mm.  In  harmony  with  the  terminology 
for  the  male  cell,  the  grown  oogonium  is  designated  a primary  oocyte.  The 
final  stages  of  oogenesis  are  maturative.  As  in  spermatogenesis,  two  cell 
divisions  take  place,  but  with  this  difference:  the  cytoplasm  is  divided 


OOGENESIS 


21 


unequally,  and  instead  of  four  cells  of  equal  size  resulting,  there  are  formed 
one  large  ripe  ovum,  or  ootid,  and  three  rudimentary  or  abortive  ova, 
known  as  polar  bodies,  or  polocytes  (Fig.  15).  The  number  of  chromosomes 


Fig.  14. — An  advanced  Graafian  follicle  and  ovum  from  a girl  of  fifteen  (Prentiss).  X 30. 


is  reduced  in  the  same  manner  as  in  the  male,  so  that  the  ripe  ovum  and 
each  polar  cell  contain  one-half  the  number  of  chromosomes  found  in  the 
oogonium  or  primary  oocyte. 


Polar  bod v / 


Centrosome 
Sperm  head 


Polar  body  II 


\ r 


sperm  tail 

. 


cJ 


A 


B 


Fig.  15. — .4,  Formation  of  the  first  polar  cell  in  the  mouse  ovum  (Sobotta).  X 1500.  B, 
Separation  of  the  second  polar  cell  in  the  bat  ovum  (after  Van  der  Stricht). 


During  maturation  the  ovum  and  first  polocyte  are  termed  secondary 
oocytes  (comparable  to  secondary  spermatocytes) ; the  mature  ovum 
(ootid)  and  second  polocyte,  with  the  daughter  cells  of  the  first  polocyte. 
are  comparable  to  the  spermatids  (Fig.  12).  Each  spermatid,  however. 


22 


THE  GERM  CELLS  AND  FERTILIZATION 


may  form  a mature  spermatozoon,  but  only  one  of  the  four  daughter  cells 
of  the  primary  oocyte  becomes  functional.  The  ovum  develops  at  the 
ex])ense  of  the  three  ])olocytes  which  are  abortive  and  degenerate  even- 
tually, though  it  has  been  shown  that  in  some  insects  the  polar  cell  may  be 
fertilized  and  segment  several  times  like  a normal  ovum.  In  most  animals, 
the  actual  division  of  the  first  polocyte  into  two  daughter  cells  is  suppressed 
(cf.  Fig.  1 5 B).  The  nucleus  of  the  ovum  after  maturation  is  known  as  the 
jcniale  pronndciis. 

Maturation  in  the  Mouse. — Typical  maturation  occurs  in  the  mouse. 
1'he  first  jiolocyte  is  formed  while  the  ovum  is  still  in  the  Graafian  follicle. 
Neither  astral  rays  nor  typical  centrosomes  have  been  observed;  the 
chromosomes  are  V-shaped.  The  finst  polar  cell  is  constricted  from  the 
ovum  and  lies  beneath  the  zona  pellucida  as  a spherical  mass  about 
25  micra  in  diameter  (Fig.  15  A).  Both  ovum  and  polar  cell  (secondary 
oocytes)  contain  20  chromosomes,  or  half  the  number  normal  for  the 
mouse.  The  first  maturation  division  is  the  reductional  one  and  the 
chromosomes  take  the  form  of  tetrads. 

After  ovulation  has  taken  place,  the  ovum  lies  in  the  ampulla  of 
the  uterine  tube.  If  fertilization  occurs,  a second  polocyte  is  cut  off,  the 
nucleus  of  the  ovum  not  having  regained  its  membrane  between  the 
production  of  the  first  and  second  polar  bodies  (Figs.  15  A and  17  A,  D). 
The  second  maturation  spindle  and  second  polar  cell  are  smaller  than  the 
first.  Immediately  after  the  appearance  of  the  second  polar  cell,  the 
chromosomes  resolve  themselves  into  a reticulum  and  the  female  pro- 
nucleus  is  complete  [Mig.  17  D). 

Maturation  in  Man. — -The  only  observations  are  those  of  Thompson 
(1919),  who  believes  to  have  identified  stages  in  the  formation  of  all  three 
polar  cells  prior  to  ovulation  or  fertilization.  The  evidence  presented, 
however,  can  hardly  be  accepted  as  conclusive.  Yet,  in  Tarsius,  a low 
primate,  both  polar  cells  have  been  observed. 

OVULATION  AND  INSEMINATION 

The  ripe  germinal  products  are  next  released  from  their  respective 
sex  glands  and  then  brought  together. 

Ovulation. — The  discharge  of  the  ovum  from  its  follicle  comprises 
ovulation.  A few  animals  breed  continuously,  but  commonly  there  is  a 
seasonal  or  annual  spawning  period.  The  several  mammalian  groups 
show  various  gradations  between  an  almost  continuous  breeding  period 
(oestrus)  and  an  annual  one.  In  man  ovulation  is  periodic,  at  intervals 
of  four  weeks,  beginning  at  puberty  and  ending  with  the  menopause. 
However,  fully  formed  Graafian  follicles  appear  in  the  ovary  during  the 
second  year  of  infancy,  and,  in  some  individuals,  even  before  birth. 


OVULATION  AND  INSEMINATION 


23 


Ovulation  may  occur  at  this  time,  but  usually  these  precociously  formed 
follicles  degenerate  with  their  contained  ova.  Generally,  only  one  follicle 
and  ovum  mature  each  month,  the  ovaries  roughly  alternating.  Yet, 
ordinary  multiple  births  depend  on  the  rupture  of  two  or  more  follicles. 
Rarely  in  man,  but  frequently  in  the  monkey,  follicles  contain  more  than 
one  egg.  Thus,  from  the  thousands  of  potential  ova,  only  about  200 
ripen  in  each  ovary  during  the  30  years  of  sexual  activity. 

The  completed  follicle  is  from  5 to  12  mm.  in  diameter.  It  makes 
a bud-like  protuberance  from  the  surface  of  the  ovary,  and  at  this  point 
the  ovarian  wall  is  very  thin  (Fig.  16  A).  Internally,  the  follicle  contains 
fluid,  probably  under  vascular  and  muscular  tension.  The  precise  factors 
which  cause  rupture  are  not  positively  known,  but  they  doubtless  include 
mechanical  pressure,  perhaps  combined  with  a weakening  of  the  follicular 
wall  by  the  digestive  influence  of  the  contained  fluid  (Schochet,  1920). 


Fig.  16. — .4,  Human  uterine  tube  and  ovary  with  mature  Graafian  follicle]  (Ribemont- 
Dessaignes).  B,  Sectioned  human  ovary  with  a corpus  luteum  verum  and  two  corpora  albi- 
cantia.  X 1.5. 

When  the  follicle  bursts,  the  fluid  gushes  out,  carrying  with  it  the 
ovum  torn  loose  from  its  cumulus  oophorus.  The  adhering  follicular 
cells,  immediately  investing  the  ovum,  constitute  the  corona  radiata  (Fig.  6). 
The  ovum  is  swept  into  the  uterine  tube  by  inwardly  stroking  cilia  of 
the  tubal  Ambriae.  Although  the  ovum  is  now  ready  to  be  fertilized,  it 
is  not  yet  technically  ‘mature,’  for  the  last  polar  division  awaits  the 
stimulus  of  fertilization. 

The  Corpus  Luteum. — After  ovulation,  a blood  clot,  the  corpus  liemorrhagicum,  forms 
within  the  empty  follicle.  The  follicle  cells  of  the  stratum  granulosum  proliferate,  enlarge, 
and  produce  a yellow  pigment.  The  w'hole  structure,  composed  of  lutein  cells  and  con- 
nective-tissue strands,  is  termed  the  corpus  luteum,  or  yellow  body  (Fig.  16  B).  If  preg- 
nancy does  not  supervene,  the  corpus  luteum  spurium  reaches  its  greatest  development 
within  two  weeks  and  then  gradually  is  replaced  by  fibrous  tissue ; the  resultant  white  scar 
is  known  as  the  corpus  albicans.  In  pregnancy  the  corpus /zhtvuKiier/oH  continues  its  growth 
until,  at  the  thirteenth  week,  it  reaches  a maximal  diameter  of  1 5 to  30  mm. ; at  term  it  is 
still  a prominent  structure  in  the  ovary.  The  corpus  luteum  is  believed  to  produce  an 


24 


THE  GERM  CELLS  AND  FERTILIZATION 


important  internal  secretion,  for  if  removed  the  ovum  fails  to  attach  to  the  wall  of  the 
uterus,  or  if  the  ovum  is  already  embedded,  development  ceases  (Fraenkel).  An  influence 
in  retarding  ovulation  and  stimulating  the  mammary  gland  function  has  also  been 
shown  experimentally  (L.  Loeb;  O’Donoghue). 

Relation  of  Ovulation  and  Menstruation. — Since  human  ovulation 
and  menstruation  both  begin  with  puberty,  recur  at  about  twenty-eight 
day  intervals,  and  discontinue  during  pregnancy  and  at  the  menopause, 
a close  relation  has  long  been  inferred.  The  cessation  of  the  menses  after 
ovarian  removal  further  indicates  dependence.  For  many  years  the  two 
processes  were  supposed  to  be  synehronous.  This  belief  was  based  upon 
clinical  oliservations  by  Leopold,  Ravano  and  others  who  tried  to  correlate 
the  ages  of  corpora  lutea  with  known  menstrual  histories.  Since  then, 
Meyer,  Ruge,  Schroder,  Fraenkel,  and  Halban,  utilizing  better  standard- 
ized corpora  lutea,  have  presented  convincing  evidence  that  ovulation 
occurs  most  often  between  the  fourth  and  fourteenth  day  after  the  men- 
strual onset.  While  correct  as  a generalization,  this  correlation  is  not 
rigid  and  often  ova  are  liberated  at  other  times.  Moreover,  in  young 
girls  ovulation  may  precede  the  inception  of  menstruation  and  it  may 
occur  in  women  during  pregnancy  and  lactation  or  after  the  menopause. 

Coitus  and  Insemination.  -In  most  aquatic  animals  the  eggs  and 
sperm  are  discharged  externally  at  about  the  same  time  and  place.  Their 
meeting  depends  largely  upon  chance,  enhanced  by  the  production  of 
immense  numbers  of  spermatozoa.  Some  animals  increase  the  certainty 
of  such  cell  union  by  a pscudocopiilation ; thus,  the  male  frog  clasps  the 
female  and  jiours  his  milt  over  the  eggs  as  they  are  extruded.  Many 
invertebrates  and  all  amniote  vertebrates  have  their  sex  cells  unite  inside 
the  female’s  body.  This  is  effected  by  the  sexual  embrace  termed  copu- 
lation,  or  coitus.  In  general,  those  animals  whose  offspring  reach  maturity 
with  reasonable  surety  (as  the  result  of  internal  fertilization  and  postnatal 
care)  produce  fewer  germ  cells,  especially  ova,  than  those  that  leave 
fertilization  to  chance  and  development  to  hazard.  The  codfish  produces 
10,000,000  eggs  in  a breeding  period,  a sea  urchin  20,000,000;  in  certain 
birds  and  mammals  only  a single  egg  is  matured,  yet  the  stock  of  each 
remains  constant. 

The  purpose  of  coitus  is  to  introduce  spermatozoa  into  the  vagina. 
The  completed  human  sperm  detach  from  the  Sertoli  cells,  and  clusters 
are  moved  along  the  efferent  ductules  into  the  epididymis.  Here  they 
become  separate  and  motile,  due  to  a secretion  of  the  duct  epithelium. 
The  seminal  fluid  accumulates  about  the  ampulla  of  the  ductus  deferens; 
its  storage  in  the  seminal  vesicles  is  much  questioned.  At  the  climax  of 
coitus  ejaculation  occurs  and  the  spermatozoa,  suspended  in  seminal 
fluid,  are  forcibly  ejected.  The  seminal  fluid,  or  semen,  is  a mixture 


FERTILIZATION 


25 


chiefly  of  the  secretions  of  the  seminal  vesicles,  prostate,  and  bulbo- 
urethal  glands,  in  which  occur  the  spermatozoa.  The  volume  of  the 
ejaculate  is  about  3 c.c.  and  in  it  swim  over  200,000,000  spermatozoa. 

The  outstanding  functional  feature  of  spermatozoa  is  their  flagellate 
swimming.  Because  of  this  they  were  once  regarded  as  parasites  living 
in  the  seminal  fluid.  Forward  progress  is  at  the  rate  of  about  2.5  mm. 
a minute,  which,  length  for  length,  compares  with  the  ordinary  gait  of 
man.  An  acid  environment,  such  as  the  vagina,  is  deleterious  or  fatal; 
an  alkaline  medium,  as  furnished  by  the  uterus,  is  favorable.  Sperma- 
tozoa tend  always  to  swim  against  feeble  currents.  This  is  important, 
as  the  outwardl^^  stroking  cilia  of  the  uterine  tubes  and  uterus  direct  the 
spermatozoa  by  the  shortest  route  to  the  ovum.  They  probably  reach 
the  ampulla  of  the  uterine  tube  two  hours  or  more  after  coitus. 

Spermatozoa  have  been  found  motile  in  the  uterine  tube  nine  days 
after  the  admission  of  a patient  to  the  clinic,  and,  according  to  her  state- 
ment, three  and  one-half  w^eeks  after  coitus.  They  have  been  kept 
alive  eight  days  outside  the  body.  It  is  not  known  for  how  long  sperma- 
tozoa are  capable  of  fertilizing  ova.  Keibel  holds  that  this  would  cer- 
tainly be  more  than  a week.  However,  Lillie  (1915)  has  shown  with  sea 
urchins  that  the  ability  to  fertilize  is  lost  long  before  vitality  or  motility 
is  impaired,  and  Mall  (1918)  concludes  that  the  duration  of  the  fertilizing 
power  of  human  spermatozoa  is  safely  less  than  the  corresponding  period  in 
the  ovum,  which  is  probably  for  fully  24  hours  after  ovulation.  In  the 
hen,  spermatozoa  remain  functional  three  weeks;  in  bats  six  months;  in 
bees  five  years. 

FERTILIZATION 

The  formation,  maturation,  and  meeting  of  the  male  and  female  germ 
cells  are  all  preliminary  to  their  actual  union  which  definitely  marks  the 
beginning  of  a new  individual.  This  penetration  of  ovum  by  sperma- 
tozoon and  the  fusion  of  their  ‘pronuclei’  constitute  the  process  oi  jertili- 
zation.  In  practically  all  animals,  fertilization  also  starts  the  ovum 
dividing  and  thus  initiates  development  in  the  ordinary  sense.  A few 
invertebrates,  however,  can  develop  without  the  aid  of  fertilization; 
this  method  is  styled  parthenogenesis,  and  in  such  eggs  there  is  usually  but 
one  polar  cell  and  hence  no  chromosome  reduction. 

Random  movements  of  the  sperm  bring  them  in  contact  with  ova. 
It  is  very  doubtful  whether  there  is  any  chemical  attraction.  In  some 
forms,  as  for  example  fishes,  tactile  response  keeps -the  spermatozoa  in 
contact  with  anything  touched.  In  mammals,  amphibia,  and  many 
invertebrates,  the  ovum  is  either  naked  or  surrounded  by  a delicate 
vitelline  membrane.  Spermatozoa  can  enter  such  eggs  at  any  point. 
Ova  that  are  invested  with  heavy  membranes  usually  have  a definite 


26 


THE  GERM  CELLS  AND  FERTILIZATION 


funnel-shaped  aperture,  the  micro pyle,  through  which  the  male  cell  must 
enter.  Only  motile  spermatozoa  are  able  to  attach  to  the  surface  of  an 
egg;  it  is  probable  that  forces  allied  to  phagocytosis,  rather  than  vibra- 
tional energy,  accomplish  the  actual  ‘penetration.  ’ 

In  general,  only  one  spermatozoon  normally  enters  an  egg;  how  others, 
endeavoring  to  penetrate,  are  thereafter  excluded  is  not  entirely  clear. 
If  accident  or  im]iaired  vitality  admits  more  than  one  sperm,  development 
is  abnormal  and  soon  ends.  On  the  contrary,  some  sharks,  amphibia, 
reptiles,  and  birds  normally  exhibit  such  polyspermy.  In  all  these  cases, 
however,  only  one  spermatozoon  unites  with  the  female  pronucleus. 

The  fertilized  ovum  derives  its  nuclear  substance  equally  from  both 
parents,  the  cytoplasm  (and  yolk)  almost  entirely  from  the  mother,  the 
centrosome  probably  from  the  father. 

The  fundamental  results  of  fertilization  are:  (i)  the  union  of  male  and 
female  pronuclei  to  form  the  cleavage  nucleus  (thus  restoring  the 
original  number  of  chromosome  pairs);  (2)  the  initiation  of  cell  division, 
■or  cleavage,  in  which  all  male  and  female  chromosomes  take  part. 

These  two  factors  are  separate  and  independent  phenomena.  It  has  been  shown  by 
Boveri  and  others  that  fragments  of  sea  urchin’s  ova  containing  no  part  of  the  nucleus  may 
be  fertilized  by  spermatozoa,  segment,  and  develop  into  larvae.  The  female  chromosomes 
are  thus  not  essential  to  the  process  of  cleavage.  Loeb,  on  the  other  hand,  proved  that 
the  ova  of  invertebrates  may  be  made  to  develop  by  chemical  and  mechanical  means 
without  the  cooperation  of  the  spermatozoon  {artificial  parthenogenesis).  Even  adult  frogs 
have  been  reared  from  mechanically  stimulated  eggs.  These  facts  show  that  the  actual 
union  of  the  male  and  female  pronuclei  is  not  the  means  of  initiating  the  development  of 
the  ova.  In  all  vertebrates  it  is,  nevertheless,  the  end  and  aim  of  fertilization. 

Lillie  maintains  that  the  cortex  of  a sea  urchin’s  ovum  produces  a substance,  fertilizin. 
This  he  regards  as  an  amboceptor  essential  to  fertilization,  with  one  side  chain  which  agglu- 
tinates and  attracts  the  spermatozoa,  and  another  side  chain  which  activates  the  cytoplasm 
and  initiates  the  cleavage  of  the  ovum.  According  to  Loeb,  agglutination  is  proved  in 
but  few  forms  and  Lillie’s  interpretation  fails  to  meet  all  the  facts.  Loeb  holds  that  the 
spermatozoon  actually  activates  the  ovum  to  develop  by  increasing  its  oxidations  and  by 
rendering  it  immune  to  the  toxic  effects  of  oxidation. 

Fertilization  in  the  Mouse. — Normally,  a single  spermatozoon 
enters  the  ovum  six  to  ten  hours  after  coitus.  While  the  second  polar  cell 
is  forming,  the  spermatozoon  penetrates  the  ovum  and  loses  its  tail 
(Fig.  17  A-C).  Its  head  enlarges  and  is  converted  into  the  male  pro- 
jiuclcits  (D).  The  pronuclei,  male  and  female,  approach  (E)  and  resolve 
first  into  a spireme  stage  (F),  then  into  two  groups  of  20  chromosomes 
(G).  A centrosome,  possibly  that  of  the  male  cell  (cf.  Fig.  15  B),  appears 
between  them,  divides  into  two,  and  soon  the  first  cleavage  spindle  is 
formed  (F-H).  The  20  male  and  20  female  chromosomes  arrange  them- 
selves in  the  equatorial  plane  of  the  spindle,  thus  making  the  original 


FERTILIZATION 


27 


number  of  40  (H).  Fertilization  is  now  complete  and  the  ovum  divides 
in  the  ordinay  way  (7,  /),  the  daughter  cells  each  receiving  equal  numbers 
of  maternal  and  paternal  chromosomes. 

Fertilization  in  Man. — The  union  of  the  human  germ  cells  is  believed 
usually  to  take  place  in  the  ampulla  of  the  uterine  tube,  although  it  never 


Fig.  17. — Fertilization  of  the  ovum  of  the  mouse  (Sobotta).  X 500.  A-D,  Entrance 
of  the  spermatozoon  and  formation  of  the  polar  cells;  D-E,  development  of  the  pronuclei ; 
F—J,  union  of  chromosomes  and  the  first  cleavage  spindle. 

has  been  observed  in  any  primate  except  Tarsius.  This  conclusion  is 
supported  by  direct  observations  on  other  mammals  and  by  the  frequency 
of  tubal  pregnancies  at  this  site.  Rarely  ova  become  fertilized  before 
entering  the  tube,  but  the  possibility  of  fertilization  after  they  have 
reached  the  uterus  is  usually  denied. 

To  be  fruitful,  the  time  of  coitus  and  ovulation  must  roughly  agree 
(p.  22),  and,  on  the  average,  about  one  day  is  supposed  to  elapse  between 
insemination  and  fertilization.  Most  conceptions  occur  during  the  week 


28 


THE  GERM  CELLS  AND  FERTILIZATION 


or  ten  days  following  menstruation;  this  is  in  harmony  with  the  known 
data  on  ovulation  time  (p.  24). 

While  there  are  no  direct  observations  on  fertilization  in  man,  the 
]irocess  has  been  studied  throughly  in  several  mammals.  In  all  essentials 
it  undoubtedly  follows  the  common  course  as  described  for  the  mouse. 

Superfetation.-- -df  an  ovum  is  liberated  by  a pregnant  woman  and 
fertilized  at  a later  coitus,  it  may  develop  into  a second,  younger  fetus. 
This  rare  condition,  called  sit perjctation,  is  often  denied,  yet  in  the  early 
weeks  of  ])regnancy  it  is  theoretically  possible.  Superfetation  should 
not  be  confused  with  strikingly  unequal  twin  development,  due  to  nutri- 
tional or  other  inequalities. 

HEREDITY  AND  SEX 

The  Significance  of  Mitosis  and  Maturation.  —The  complicated  processes  of  mitosis 
serve  the  purpose  of  dividing  accurately  the  chromatic  substance  of  the  nucleus  in  such  a 
way  that  the  self-pcrjietuating  chromosomes  of  each  daughter  cell  may  be  the  same,  both 
quantitatively  and  ciualitatively.  This  is  important  since  it  is  believed  by  most  students  of 
heredity  that  chromatin  particles,  or  genes,  in  the  chromosomes  bear  the  hereditary  char- 
acters, and  that  these  are  arranged  in  definite  linear  order  in  particular  chromosomes.  At 
maturation  there  is  a side  by  side  union  of  like  chromosomes,  one  member  of  each  pair 
having  come  from  the  father,  the  other  from  the  mother  of  the  preceding  generation; 
each  member,  however,  carries  the  same  general  set  of  hereditary  charaeters  as  its  mate. 
At  this  stage  of  chromosomal  conjugation  there  may  be  an  interchange,  or  ‘crossing  over,’ 
of  corresponding  genes,  resulting  in  new  hereditary  combinations.  The  reducing  division 
of  maturation  separates  whole  chromosomes  of  each  pair,  but  chance  alone  governs  the 
actual  assortment  of  paternal  and  maternal  members  to  the  daughter  cells;  this  mitosis 
obviously  halves  the  chromosome  number  characteristic  for  the  species.  The  significance 
of  the  ecjuational  maturation  mitosis,  beyond  accomplishing  mere  cellular  multiplication, 
is  obscure. 

Mendel’s  Law  of  Heredity. — E.xperiments  show  that  hereditary  characters  fall  into 
two  opposing  groups,  the  contrasted  pairs  of  which  are  termed  allelomorphs.  As  an 
example,  we  may  take  the  hereditary  tendencies  for  dark  and  blue  eyes.  It  is  believed  that 
there  are  paired  chromatic  particles,  or  genes,  which  are  responsible  for  these  hereditary 
tendencies,  and  that  paired  spermatogonial  chromosomes  bear  one  each  of  these  genes. 
Each  chromosome  pair  in  separate  germ  cells  may  possess  similar  genes,  both  bearing  dark- 
eyed tendencies  or  both  blue-eyed  tendencies,  or  opposing  genes,  bearing  the  one  dark-, 
the  other  blue-eyed  tendencies.  It  is  assumed  that  at  maturation  these  paired  genes  are 
separated  along  with  the  chromosomes,  and  that  one  only  of  each  pair  is  retained  in  each 
germ  cell. 

In  our  example,  either  a blue-eyed  or  a dark-eyed  tendency-bearing  particle  would  be 
retained.  At  fertilization,  the  segregated  genes  of  one  sex  may  enter  into  new  combina- 
tions with  those  from  the  other  sex.  Three  combinations  are  possible.  If  the  color  of  the 
eyes  be  taken  as  the  hereditary  character:  (i)  two  ‘dark’  germ  cells  may  unite;  (2)  two 
‘blue’  germ  cells  may  unite;  (3)  a ‘dark’  germ  cell  may  unite  with  a ‘blue’  germ  cell.  The 
offspring  in  (i)  will  all  have  dark  eyes,  and,  if  interbred,  their  progeny  will  likewise  inherit 
dark  eyes  exclusively.  Similarly,  the  offspring  in  (2),  and  if  these  are  interbred  their 
progeny  as  well,  will  include  nothing  but  blue-eyed  individuals.  The  first  generation  from 
the  cross  in  (3)  will  have  dark  eyes  solely,  for  black  in  the  present  example  is  dominant,  as 


HEREDITY  AND  SEX 


29 


it  is  termed.  Such  dark-eyed  individuals,  nevertheless,  possess  both  dark-  and  blue- 
eyed bearing  genes  in  their  germ  cells;  in  the  progeny  resulting  from  the  interbreeding  of 
this  class,  the  original  condition  is  repeated — pure  darks,  impure  darks  which  hold  blue 
recessive,  and  pure  blues  will  be  formed  in  the  ratio  of  1:2:1  respectively.  It  is  thus  seen 
that  blue-eyed  children  may  be  born  of  dark-eyed  parents,  whereas  blue-eyed  parents 
can  never  have  dark-eyed  offspring.  Many  such  allelomorphic  pairs  of  hereditary  char- 
acters are  known. 

Cytoplasmic  Inheritance. — Certain  eggs  show  distinct  cytoplasmic  zones  which 
cleavage  later  segregates  into  groups  of  cells  destined  to  form  definite  organs  or  parts. 
In  a sense  this  represents  a refined  sort  of  preformation,  but  prelocalization  is  a more  exact 
term.  From  these  facts  Conklin  and  Loeb  argue  that  the  cytoplasm  is  really  the  embryo 
in  the  rough,  the  nucleus,  through  Mendelian  heredity,  adding  only  the  finer  details. 
Morgan,  among  others,  refuses  to  admit  the  validity  of  this  interpretation. 

The  Determination  of  Sex. — The  sex-determining  power  lies  in  a chromosome  that  can 
be  identified  in  many  animals.  This  chromosome  is  termed  the  accessory,  X,  or  sex 
chromosome.  According  to  Painter  (1923),  humair  obgonia  contain  46  ordinary  chromo- 
somes and  two  X -chromosomes.  At  maturation  the  number  is  halved,  and  all  oocytes  and 
polocytes  contain  23  -f  X.  The  spermatogonia,  on  the  contrary,  contain  46  ordinary 
chromosomes,  one  X-chromosome  and  its  diminutive  mate,  called  the  Y-chromosome. 
After  maturation,  therefore,  half  the  spermatids  have  23  -j-  X,  the  remaining  half  have 
23  -p  y.  When  a spermatozoon  with  23  + X fertilizes  an  ovum,  the  number  is  restored 
to  46  -p  2X  and  a female  results.  When  a spermatozoon  with  23  -p  Y fertilizes,  the  out- 
come is  46  -p  X -p  Y and  a male  results. 

Many  animals  lack  the  Y,  and  the  male  cells  contain  an  odd  number  of  chromosomes. 
Reduction  then  forms  two  classes  of  spermatozoa,  those  with  the  extra  chromosome  being 
female  producing.  In  certain  birds  and  moths  the  system  is  the  exact  reverse,  inasmuch 
as  the  spermatozoa  are  all  alike  in  chromosomal  constitution  while  the  eggs  are  of  two  sorts. 


CHAPTER  II 


CLEAVAGE  AND  THE  ORIGIN  OF  THE  GERM  LAYERS 

CLEAVAGE 

The  fertilized  ovum  ])romptly  begins  to  form  the  new,  multicellular 
individual  by  a process  termed  cleavage,  or  segmentation . This  comprises 
orderly  and  rapid  successions  of  mitoses  which  result  in  an  aggregate  of 
smaller  cells,  called  blastomercs.  Every  blastomere  receives  the  full 
assortment  of  chromosomes,  half  from  each  parent  (Fig.  17  F-J). 

The  abundance  and  distribution  of  yolk  in  the  egg  so  influences 
mitosis  as  to  allow  the  following  classification  of  cleavage: 

(A)  Total.  Entire  ovum  divides;  holoblastic  ova. 

1.  Equal.  In  isolecithal  ova;  blastomeres  are  of  equal  size; 
e.g.,  amphioxus  and  mammals. 

2.  Unequal.  In  moderately  telolecithal  ova;  yolk  accumulated 
at  vegetal  pole  retards  mitosis,  and  fewer  but  larger  blast- 
omeres form  there;  e.g.,  lower  fishes  and  amphibia. 

{B)  Partial.  Protoplasmic  regions  alone  cleave;  meroblastic  ova. 

1.  Discoidal.  In  highly  telolecithal  ova;  mitosis  restricted  to 
anim^d  ]Jole;  e.g.,  higher  fishes,  reptiles,  and  birds. 

2.  Superficial.  In  centrolecithal  ova;  mitosis  restricted  to  the 
peri])heral  cytoplasmic  investment ; arthropods. 

Cleavage  in  Amphioxus. — The  early  processes  of  development  are 
easily  understood  in  a primitive,  fish-like  form,  Amphioxus.  About  one 
hour  after  fertilization,  its  essentially  isolecithal  ovum  divides  vertically 
into  two  nearly  equal  blastomeres  (Fig.  18,  2).  Within  the  next  hour  the 
daughter  cells  again  cleave  in  the  vertical  plane,  at  right  angles  to  the 
first  division,  thus  forming  four  cells  (3).  Fifteen  minutes  later  a third 
division  takes  place  in  a horizontal  plane  (4).  As  the  yolk  is  somewhat 
more  abundant  at  the  vegetal  poles  of  the  four  cells,  the  mitotic  spindles 
lie  nearer  the  animal  pole.  Consequently,  in  the  eight-celled  stage  the 
upper  tier  of  four  cells  is  slightly  smaller  than  the  lower  four.  By  suc- 
cessive cleavages,  first  in  the  vertical,  then  in  the  horizontal  ]3lane,  a 
16-  and  3 2 -celled  embryo  is  formed  (5,  6).  The  upper  two  tiers  are  now 
smaller,  and  a cavity,  the  blastocode,  is  enclosed  by  the  cells.  The  embryo 
at  this  stage  is  sometimes  called  a morula  because  of  its  resemblance  to  a 
mulberry.  In  subsequent  cleavages,  as  development  proceeds,  the  size 

30 


CLEAVAGE 


31 


of  the  cells  is  diminished,  while  the  cavity  enlarges  (7,  8).  The  embryo 
is  now  a blastnla,  nearly  spherical  in  form  and  about  four  hours  old.  The 
cleavage  of  the  holoblastic  Amphioxus  ovum  is  thus  total  and  nearly 
equal. 


Fig.  18. — Cleavage  in  Amphioxus,  viewed  laterally  (Hatschek).  X 200.  i.  Mature  egg, 
with  one  polar  body  (P.5.) ; the  other  missing.  2.  Ovum  partly  divided  into  two  blastomeres. 
3.  Four  blastomeres.  4.  Eight  blastomeres.  5.  Sixteen  blastomeres.  6.  Thirty-two  blasto- 
meres, hemisected  to  show  the  blastocoele,  B.  7,  8.  Total  and  hemisected  blastul®. 

Cleavage  in  Lower  Fishes  and  Amphibia. — These  ova  contain  enough 
yolk  so  that  the  nucleus  and  most  of  the  cytoplasm  lie  nearer  the  upper, 
or  animal  pole.  The  first  cleavage  spindle  appears  eccentrically  in  this 
cytoplasm.  The  first  two  cleavage  planes  are  vertical  and  at  right  angles, 
and  the  four  resulting  cells  are  equal.  The  spindles  for  the  third  cleavage 
are  located  near  the  animal  pole,  and  the  division  takes  place  in  a hori- 

e H\Uii  H ^ be  ^ ^ ^ ^ 


32 


CLEAVAGE  AND  THE  ORIGIN  OF  THE  GERM  LAYERS 


Fig.  20. — Cleavage  of  the  pigeon’s  ovum  (redrawn  from  Blount).  A,  Blastoderm  in  surface 

view;  B,  in  vertical  section. 

vertical  but  the  inert  yolk  does  not  cleave.  The  segmentation  is  thus 
partial  and  discoidal.  In  the  bird’s  ovum,  the  cytoplasm  is  divided  by 
successive  vertical  furrows  into  a mosaic  of  cells,  which,  as  it  increases  in 
size,  forms  a cap-like  structure  upon  the  surface  of  the  yolk  (Fig.  20  A). 
These  cells  are  separated  from  the  yolk  beneath  by  horizontal  cleavage 


zontal  plane.  As  a result,  the  upper  four  cells  are  much  smaller  than  the 
lower  four  (Fig.  ig  A).  The  large,  yolk-laden  cells  divide  more  slowly 
than  the  upper,  small  cells  (B-D).  At  the  blastula  stage,  the  cavity  is 
small,  and  the  cells  of  the  vegetal  pole  are  many  times  larger  than  those 
of  the  animal  pole  {E,  F).  The  cleavage  is  thus  total  but  unequal. 


Fig.  19. — Cleavage  and  gastrulation  in  the  frog.  X 12.  A-D,  Cleavage  stages;  E, 
blastula;  F,  hemisection  of  E;  G,  early  gastrula;  II,  hemisection  of  G.  an.,  Animal  cells;  arch., 
archenteron;  b'c.,  blastocoele;  b’p.,  blastopore;  ect.,  ectoderm;  ent.,  entoderm;  v’g.,  vegetal  cells. 


Cleavage  in  Higher  Fishes,  Reptiles  and  Birds. — The  ova  of  these 
vertebrates  contain  a large  amount  of  yolk.  There  is  very  little  pure 
cytoplasm  except  at  the  animal  pole,  and  here  the  nucleus  is  located 
(Fig.  6).  When  segmentation  begins,  the  first  plane  of  separation  is 


Blastomere  Blastoc(Ble 


'itelline  membrane 


Zo)ui 

Pi'tbirida 


Polar  bodies 


Tiobherioilenn 


Manila . 


Pig.  21. — Diagrams  of  cleavage  ami  the  blastodermic  vesicle  in  the  raiit^it  (Thomson,  after 

van  Beneden).  X 200. 


CLEAVAGE 


33 


furrows,  and  successive  horizontal  cleavages  give  rise  to  several  layers  of 
cells  (Fig.  20  B).  The  space  between  cells  and  yolk  mass  may  be  com- 
pared to  the  blastula  cavity  of  Amphioxus  and  the  frog  (Fig.  22).  The 
cellular  cap  is  termed  the  germinal  disc,  or  blastoderm.  The  yolk  mass, 
which  forms  the  floor  of  the  blastula  cavity  and  the  greater  part  of  the 
ovum,  may  be  compared  to  the  large,  yolk-laden  cells  at  the  vegetal  pole 
of  the  frog’s  blastula.  The  main  yolk  mass  never  divides  but  is  gradually 
used  up  in  supplying  nutriment  to  the  embryo  which  is  developed  from  the 
cells  of  the  germinal  disc.  At  the  periphery  of  the  blastoderm,  new  cells 
form  progressively  until  they  enclose  the  yolk  (Fig.  22  C). 

Cleavage  in  Mammals. — -The  ovum  of  all  the  higher  mammals, 
including  man,  is  isolecithal  and  nearly  microscopic  in  size.  Its  cleavage 


Fig.  22. — Diagrams  of  blastula  homologies  (Prentiss).  .4,  Amphioxus;  B,  frog;  C,  chick; 

D,  mammal. 

has  been  studied  in  several  forms,  but  the  rabbit’s  ovum  will  serve  as  an 
example.  The  cleavage  is  complete  and  nearly  equal  (Fig.  21),  a cluster 
of  approximately  uniform  cells  being  formed  within  the  zona  pellucida. 
This  corresponds  to  the  morula  stage  of  Amphioxus.  Next,  an  inner 
mass  of  cells  is  formed  that  is  equivalent  to  the  germinal  disc,  or  blasto- 
derm of  the  chick  embryo.  The  inner  cell  mass  is  overgrown  by  an  outer 
layer  which  is  termed  the  trophectoderm,  because  it  later  supplies  nutri- 
ment to  the  embryo  from  the  uterine  wall.  Fluid  then  appears  between 
the  outer  layer  and  the  inner  cell  mass,  thereby  separating  the  two  except 
3 


34 


CLE..WAGE  AKD  THE  ORIGIN  OF  THE  GERM  LAYERS 


at  the  animal  pole.  As  the  fluid  increases  in  amount,  a hollow  blasto- 
dermic vesicle  results,  its  walls  composed  of  the  single-layered  trophecto- 
derm,  except  where  this  is  in  contact  with  the  inner  cell  mass.  It  is  usually 
spherical  or  ovoid  in  form,  as  in  the  rabbit,  and  probably  such  is  the  form 
of  the  human  ovum  at  this  stage.  In  the  rabbit,  the  vesicle  is  4.5  mm. 
long  before  it  becomes  embedded  in  the  wall  of  the  uterus;  among  ungu- 
lates, or  hoofed  animals,  the  vesicle  is  greatly  elongated  and  attains  a 
length  of  several  centimeters,  as  in  the  pig. 

Comparing  the  mammalian  blastodermic  vesicle  with  the  blastula 
stages  of  Amphioxus,  the  frog,  and  the  bird,  it  will  be  seen  that  it  is  to  be 
homologized  with  the  bird’s  blastula,  not  with  that  of  Amphioxus  (Fig.  22). 
In  each  case  there  is  an  inner  cell  mass  of  the  germinal  disc.  The  troph- 
ectoderm  of  the  mammal  represents  a ])recocious  development  of  cells, 
which,  in  the  bird,  later  enveloji  the  yolk.  The  cavity  of  the  vesicle  is 
to  be  compared,  not  with  the  blastula  cavity  of  Amphioxus  and  the  frog, 
but  with  the  yolk  mass  pins  the  cleft-like  blastocoele  of  the  bird's  ovum.  The 
higher  mammalian  ovum,  although  almost  devoid  of  yolk,  thus  develops  a 
‘blastula’  resembling  that  attained  by  the  yolk-laden  ova  of  reptiles  and 
birds.  That  this  similarity  has  an  evolutionary  significance  is  attested 
by  discoidal  cleavage  in  the  highly  telolecithal  eggs  of  present-day  mono- 
treme  mammals. 

In  the  low  primate  Tarsius,  cleavage  and  the  blastodermic  vesicle 
are  well  known.  A four-celled  Macacus  ovum,  with  blastomeres  nearly 
equal  and  oval  in  form,  is  the  only  cleavage  stage  yet  observed  among 
higher  jirimates.  In  all  placental  mammals,  segmentation  of  the  ovum 
occurs  during  its  passage  down  the  uterine  tube. 

THE  FORMATION  OF  ECTODERM  AND  ENTODERM  (GASTRULATION) 

The  blastula  and  early  blastodermic  vesicle  show  no  differentiation 
into  layers.  Such  differentiation  next  takes  place,  giving  rise  first  to 
the  ectoderm  and  entoderm,  and  finally  to  the  mesoderm.  From  these  three 
primary  germ  layers  all  tissues  and  organs  of  the  body  are  derived. 

The  processes  of  gastridation,  by  which  ectoderm  and  entoderm  arise, 
and  of  mesoderm  formation  will  be  treated  separately. 

Amphioxus  and  Amphibia. — The  larger  cells  at  the  vegetal  pole 
of  the  Amphioxus  blastula  fold  inward  (Fig.  23  A,  B).  Eventually,  these 
invaginating  cells  obliterate  the  blastula  cavity  and  come  in  contact  with 
the  outer  layer  (Fig.  23  C).  The  new  cavity,  thus  formed,  is  the  primitive 
gut,  or  archenteron,  and  its  narrowed  mouth  is  the  blastopore.  The  outer 
layer  of  cells  is  the  ectoderm,  the  inner,  newly  formed  layer  is  the  entoderm. 
The  entodermal  cells  are  henceforth  concerned  in  the  nutrition  of  the 
body.  The  embryo  is  now  termed  a gastrula  (little  stomach). 


THE  FORMATION  OF  ECTODERM  AND  ENTODERM  (gASTRELATIOn) 


35 


In  amphibia,  invagination  begins  at  the  junction  of  animal  and 
vegetal  cells  (Fig.  ig  G).  Externally,  the  blastopore  appears  as  a cres- 
centic groove.  Since  the  vegetal  cells  are  large  and  the  blastocoele 
is  relatively  small,  simple  invagination  fails.  Hence,  archenteron  forma- 
tion is  aided  by  a lip-like  overgrowth  of  rapidly  dividing  cells  from  the 
animal  pole  (Fig.  ig  H). 


Animal  cells 


Vegetal  cells 

A B 

Fig.  23. — Gastrulation  in  Amphioxus.  X 200. 


Blastopore 

C 

A,  Blastula;  B,  C,  early  and  late  gastrulse. 


Blastocoele 


Entoderm 
Archenteron  ^ 


Ectoderm 


Reptiles  and  Birds. — ^The  germinal  disc,  or  blastoderm,  in  these 
animals  lies  like  a cap  on  the  surface  of  inert  yolk  (Fig.  6).  Since  the 
enormous  amount  of  yolk  makes  gastrulation  as  in  Amphioxus  and 
amphibians  impossible,  the  process  exhibits  marked  modifications. 

There  appears  caudally  on  the  blastoderm  of  reptiles  a pit-like  depres- 
sion. From  this  invagination,  a proliferation  of  cells  forms  a layer  which 


Invaginated 

Yolk  Ectoderm  Blastocoele  Archenteron  entoderm  Blastopore 


Cephalad  

^ (j)  (£> 

<5K3G!®3<3®  a 

AG. 

Fig.  24. — Gastrulation  in  the  pigeon,  as  shown  by  a longitudinal  section  of  the  blastoderm 

(redrawn  after  Patterson).  X 50. 


spreads  beneath  the  ectoderm.  The  inner  layer,  originating  in  this 
manner,  is  the  entoderm,  and  the  region  of  the  pit,  where  ectoderm  and 
entoderm  are  continuous,  is  the  blastopore.  In  Fig.  27  A these  changes 
are  complete. 

In  birds,  the  caudal  portion  of  the  blastoderm  is  rolled  or  tucked 
under,  the  inner  layer  formed  in  this  way  constituting  the  entoderm  (Fig. 
24).  The  marginal  region,  where  ectoderm  and  entoderm  meet,  bounds 
the  blastopore,  while  the  space  between  entoderm  and  yolk  is  the 
archenteron. 


36 


C'LF-AVAGE  AND  THE  ORIGIN  OF  THE  GERM  LAYERS 


Mammals.  -Cells  on  the  under  surface  of  the  inner  cell  mass  become 
arranged  in  a definite  sheet,  the  entoderm  (Fig.  2^  A).  It  is  usually  said 
to  arise  by  s])litting,  or  delamination,  although  there  are  attempts  to 
prove  ingrowth  from  a ‘blastopore.’  In  most  mammals,  the  entoderm 
spreads  rapidly  and  lines  the  blastodermic  vesicle  (Fig.  38)  but  in  Tarsius, 

Eiitodermal  (Yolk)  sac  Embryonic  ectoderm 


Pig  23. — Gastrulation  in  the  low  iirimate,  Tarsius,  as  demonstrated  by  sections  of  the  blasto- 
dermic vesicle  (redrawn  after  Hubrecht).  X 260. 

the  entoderm  forms  a much  smaller  sac  (Fig.  25  B,  C).  The  youngest 
human  embryos  known  (Fig.  40)  indicate  a previous  origin  of  entoderm 
much  as  in  Tarsius. 

ORIGIN  OF  THE  MESODERM,  NOTOCHORD  AND  NEURAL  TUBE 
Amphioxus  and  Amph  bia. — The  dorsal  portion  of  the  inner  sheet, 
which  forms  the  roof  of  the  archenteron  in  Amphioxus,  gives  rise  to 

ret. 


Fig.  26. — Origin  of  the  mesoderm  in  Amphioxus  (Hatschek).  X 425.  a/..  Lumen  of  gut; 

r//.,  notochord : ai\,  ccelom;  crrl.p.,  coelomic  pouch;  ect.,  ectoderm;  ent.,  entoderm;  n.c.,  neural 
canal;  n.g.,  neural  groove. 


paired,  lateral  diverticula,  the  ccdoniic  pouches  (Fig.  26).  These  separate 
both  from  a mid-dorsal  plate  of  cells  (the  future  notochord),  and  from 


ORIGIN  OF  THE  MESODERM,  NOTOCHORD  AND  NEURAL  TUBE 


37 


the  entoderm  of  the  gut,  and  become  the  primary  mesoderm.  The  meso- 
dermal pouches  grow  ventrad  and  their  cavities  form  the  coelom,  or  body 
cavity.  Their  outer  layers,  with  the  ectoderm,  constitute  the  body  wall, 
or  somatopleure;  their  inner  layers,  with  the  gut  entoderm,  form  the  intes- 
tinal wall,  or  splanchnopleiire.  In  the  meantime,  a dorsal  plate,  cut  off 
from  the  ectoderm,  folds  into  the  neural  tube  (anlage  of  the  nervous 
system),  and  the  notochordal  plate  becomes  a cord,  or  cylinder,  of  cells 
(axial  skeleton)  extending  the  length  of  the  embryo.  In  this  simple 
fashion  the  ground  plan  of  the  chordate  body  is  attained. 


Ectoderm 


Ectoderm 


Fig.  27. — Longitudinal  sections  of  the  snake’s  blastoderm,  at  various  stages,  to  show  the  origin 
of  the  notochordal  plate  (adapted  after  Hertwig). 

In  amphibia,  solid  mesodermal  plates  arise  in  a similar  location  and 
extend  laterally  between  the  ectoderm  and  entoderm.  Later,  these 
plates  split  into  two  layers  and  the  cavity  so  formed  is  the  coelom  (cf. 

3S)'  The  notochord  also  originates  as  in  Amphioxus. 

Reptiles, — The  same  pocket-like  depression  in  the  caudal  portion  of 
the  blastoderm,  that  gave  rise  to  the  cells  of  the  entodermal  layer,  now 
invaginates  more  extensively  and  forms  a pouch  which  pushes  forward 
between  ectoderm  and  entoderm  (Fig.  27  A and  B).  The  size  of  the 
invagination  cavity  varies  in  different  species;  in  some  it  is  elongate  and 


38 


CLEAVAGE  AND  THE  ORIGIN  OF  THE  GERM  LAYERS 


narrow,  lieing  confined  to  the  middle  line  of  the  blastoderm.  The  floor  of 
this  pouch  soon  fuses  with  the  underlying  entoderm,  and  the  two  thin, 
rupture,  and  disappear,  thus  putting  the  cavity  of  the  pouch  temporarily 
in  communication  with  the  space  (archenteron)  beneath  the  entoderm 
(Fig.  27  C).  The  cells  of  the  roof  persist  as  the  notochordal  plate,  which 
later  liccomes  the  notochord.  The  neural  folds  arise  before  the  mouth 


Ectoderm  Mesoderm 


Fig.  28. — Transverse  section  of  a snake’s  blastoderm,  at  a level  corresponding  to  the  middle  of 

Fig.  27  C (adapted  after  Hertwig). 


of  the  pouch  (blastopore)  closes,  and,  fusing  to  form  the  neural  tube, 
incorporate  the  blastopore  into  its  floor.  This  temporary  communica- 
tion between  the  neural  tube  and  the  primitive  enteric  cavity  is  the  neuren- 
teric  canal  (cf.  Fig.  27  C)  \ it  is  found  in  all  the  vertebrate  groups  (cf. 
Fig.  58).  A transverse  section  through  the  invaginated  pouch,  at  the 
time  of  rupture  of  its  floor,  and  through  the  underlying  entoderm  will 
make  clear  the  lateral  extent  of  these  changes  (Fig.  28). 

From  about  the  blastopore,  and 
from  the  walls  of  the  pouch,  mesodermal 
plates  arise  and  extend  like  wings  be- 
tween the  ectoderm  and  entoderm 
(Fig.  28).  As  in  amphibia,  they  later 
separate  into  outer  (somatic)  and  inner 
(splanchnic)  layers  enclosing  the 
Primitive  groove  coelom.  The  relation  between  noto- 
chordal plate,  mesoderm,  and  en- 
toderm, shown  in  Fig.  28,  resembles 
strikingly  the  conditions  in  Amphioxus 
(Fig.  26  A). 

Birds.- — Due  to  the  modified  gas- 
trulation  in  reptiles,  birds,  and  mam- 
mals through  the  influence  of  yolk,  a 
structure  known  as  the  primitive  streak  becomes  important.  An  account 
of  its  formation  and  significance,  based  on  conditions  found  in  the  bird, 
may  be  introduced  conveniently  at  this  place. 

Shortly  after  the  formation  of  entoderm,  an  opaque  band  appears  in 
the  median  line  at  the  more  caudal  portion  of  the  blastoderm  (Fig.  29). 


Area  opaca 

Primitive  knot 
Primitive  pit 

1 Primitive  fold 


Area  pcUiicida 
J Blood  island 


Fig.  29. — Bla.stoderm  of  a chick  cmliryo 
at  the  stage  of  the  primitive  streak.  X 20. 


ORIGIN  OF  THE  MESODERM,  NOTOCHORD  AND  NEURAL  TUBE 


39 


Along  this  primitive  streak,  which  is  at  first  merely  a linear  ectodermal 
thickening,  there  forms  a shallow  primitive  groove,  and  at  its  forward  end 
the  streak  ends  in  a knob,  the  primitive  knot,  or  node  (of  Hensen).  The 
primitive  streak  becomes  highly  significant  when  interpreted  in  the 
light  of  the  theory  of  concrescence,  a theory  of  general  application  in  ver- 
tebrate development.  It  will  be  remembered  that  the  entoderm  of  birds 
arises  by  a rolling  under  of  the  outer  layer  along  the  caudal  margin  of 
the  blastoderm.  As  the  blastoderm  expands,  it  is  believed  that  a middle 
point  on  this  margin  remains  fixed  (Fig.  30  A)  while  the  edges  of  the 
margin  on  eaeh  side  are  carried  caudad  and  brought  together  (B,  C). 
Thus,  a crescentic  margin  is  transformed  into  a longitudinal  slit.  Since 


ABC  D 


Fig.  30. — Diagrams  to  illustrate  the  formation  of  the  primitive  streak  according  to  the  theorj- 
of  concrescence.  The  expanding  blastoderm  is  indicated  by  dotted  circles. 

this  marginal  lip  originally  bounded  the  blastopore  (p.  35),  the  longitudinal 
slit  must  also  be  an  elongated  blastopore  whose  direction  has  merely  been 
changed.  The  lips  of  the  slit  fuse,  forming  the  primitive  streak  (D). 
The  teachings  of  comparative  embryology  support  these  conclusions,  for 
the  neurenteric  canal  arises  at  the  cranial  end  of  the  primitive  streak,  the 
anus  at  its  caudal  end,  while  the  primary  germ  layers  fuse  in  its  substance. 
All  these  relations  exist  at  the  blastopore  of  the  lower  animals. 


Ectoderm 


Mesoderm 


Head  fold 
Ecto- 
derm 


\^eiiral  plate  Primitive  knot 

imitive  bit 


Primitive  streak 


Fig.  31. — Median  longitudinal  section  of  a chick  embryo  at  the  stage  of  the  primitive  streak 

and  head  process.  X 100. 


From  the  thickened  ectoderm  of  the  primitive  streak  a proliferation 
of  cells  takes  place,  and  there  grows  out  laterally  and  caudally  between 
the  ectoderm  and  entoderm  a solid  plate  of  mesoderm  which  soon  splits 
into  somatic  and  splanchnic  layers  (Fig.  316).  An  axial  growth,  the 
head  process,  or  notochordal  plate,  likewise  extends  forward  from  the  primi- 
tive knot  and  fuses  at  once  with  the  entoderm  (Figs.  31  and  317).  Since 
the  primitive  streak  represents  a modified  blastopore,  it  is  evident  that 


40 


CLEAVAGE  AND  THE  ORIGIN  OF  THE  GERM  LAVERS 


this  cranial  extension,  the  head  process,  corresponds  to  the  pouch-like 
invagination  concerned  in  the  formation  of  notochord  and  mesoderm  in 
reptiles.  In  birds,  the  fusion  of  the  head  process  with  the  entoderm,  the 
relation  of  mesodermal  sheets  to  it  laterally,  the  formation  of  the  noto- 
chord from  its  tissue  and  the  occasional  traces  in  it  of  a cavity  continuous 
with  the  primitive  pit  (that  is,  a notochordal  canal),  all  recall  the  condi- 


Fig.  32. — .1,  Embryonic  disc  of  the  Mateer  human  embryo,  at  the  stage  of  the  primitive 
streak  (after  Streeter).  X 50.  B,  Embryonic  disc  of  the  Ingalls  human  embryo,  with  primi- 
tive streak  and  head  process.  X 26. 


tions  described  for  the  less  modified  invagination  in  reptiles.  The  primi- 
tive groove  is  the  visible  result  of  mesoderm  proliferation  from  the  tissue 
of  the  streak. 

Mammals.-  -A  typical  primitive  streak  appears  on  the  blastoderm 
of  mammals  (Fig.  32  A).  The  under  side  of  its  ectodermal  thickening 


Ectoderm  Primitive  groove  Mesoderm 


Fig.  33. — Transverse  section  through  the  primitive  streak  of  the  Mateer  human  embryo  (Fig. 
32  ,1 ) to  show  the  origin  of  mesoderm  (redrawn  after  Streeter).  X 185. 

proliferates  mesodermal  cells  which  grow  laterally  and  caudally  (Fig.  33). 
All  three  germ  layers  fuse  in  the  primitive  knot  and  from  it  a head  process 
soon  extends  forward  (Fig.  32  B). 

The  head  process  of  many  mammalian  embryos  contains  a cavity 
{notochordal  canal),  which  in  some  cases  is  of  considerable  size,  opening  at 


ORIGIN  OF  THE  MESODERM,  NOTOCHORD  AND  NEURAL  TUBE 


41 


the  primitive  pit  (Fig.  34).  As  in  reptiles,  the  floor  of  this  cavity  fuses 
with  the  entoderm,  and  the  two  rupture  and  disappear.  Portions  of  the 
floor,  still  persistent,  are  shown  in  Fig.  34.  Thus  a canal,  later  enclosed 
by  the  neural  folds,  and  then  known  as  the  neurenteric  canal,  puts  the  dorsal 
surface  of  the  blastoderm  in  communication  with  the  enteric  cavity 
beneath  the  entoderm  (Figs.  57  and  58).  The  roof  of  the  head  process, 
or  notochordal  plate,  is  for  a time  associated  closely  with  the  lateral  meso- 

Ectoderm  Notochordal  plate  Primitive  streak 


Remnant  of  canal  floor  Notochordal  canal  Entoderm 

Fig.  34. — Median  sagittal  section  through  the  primitive  knot  and  head  process  of  the  Ingalls 

human  embryo  (Fig.  32  5).  X 200. 

derm  (compare  these  relations  in  reptiles.  Fig.  28),  but  eventually  it 
becomes  the  notochord. 

The  mesoderm  grows  rapidly  around  the  wall  of  the  blastodermic 
vesicle,  until  Anally  the  two  wings  fuse  ventrally.  The  single  sheet  then 
splits  into  two  layers,  the  cavity  between  being  the  coelom,  or  body  cavity 

Primitive  streak  Mesodermal  segment  Neural  tube 


Fig.  35. — Diagrammatic  transverse  sections  of  mammalian  embryos.  .4,  The  origin  and 
spread  of  mesoderm  (Brjme);  B,  the  further  differentiation  of  mesoderm,  and  the  body  plan 
(Prentiss). 


(Fig.  35).  The  outer  mesodermal  layer  (somatic),  with  the  ectoderm, 
forms  the  somatopleure,  or  body  wall;  the  inner  splanchnic  layer,  with  the 
entoderm,  forms  the  intestinal  wall,  or  splanchnoplciire.  The  neural  tube 
having  in  the  meantime  arisen  from  the  neural  folds  of  the  ectoderm,  there 
is  present  the  ground  plan  of  the  vertebrate  body,  the  same  in  man  as  in 
Amphioxus  (Fig.  35  £>). 


42 


CLEAVAGE  AXD  THE  ORIGIN  OF  THE  GERM  LAYERS 


Mesoderm,  but  not  a coelom,  is  already  present  in  the  youngest  human 
embryo  yet  examined  (Fig.  40  A).  In  Tarsius,  a low  primate,  the  meso- 
derm has  two  sources:  (i)  From  the  splitting  of  ectoderm  at  the  caudal 
edge  of  the  blastoderm;  this  constitutes  the  extra-embryonic  mesoderm  and 
takes  no  part  in  forming  the  body  of  the  embryo.  (2)  The  intra-embryonic 
mesodertu,  which  gives  rise  to  body  tissues,  takes  its  origin  from  the 
primitive  streak  as  in  the  chick  and  lower  mammals.  The  origin  in  the 
human  embryo  is  probably  much  the  same  as  in  Tarsius. 

Homologies  of  Mesoderm  and  Notochord.  —In  Amphioxus  and  amphibia,  transverse 
sections  (Fig.  26)  apparently  show  that  the  mesoderm  and  notochord  are  folded  directly 
from  dorsal  gut-entoderm.  Yet  such  is  illusory,  for  the  roof  of  the  archenteron  grows  from 
the  dorsal  lip  of  the  blastopore.  Longitudinal  sections  prove  that  as  the  embryo  elongates, 
this  caudal,  formative  region  progressively  adds  to  the  roof  of  the  primitive  gut.  Hence, 
both  notochord  and  mesoderm  originate  from  the  indifferent  tissue  at  the  blastopore  where 
ectoderm  and  entoderm  meet.  In  reptiles,  birds,  and  mammals  the  mesoderm  arises  as 
lateral  proliferations  from  the  primitive  streak,  whereas  the  notochordal  plate  (head 
process)  is  a ‘growth’  from  its  anterior  end  (cf.  p.  40).  But,  as  the  primitive  streak  is  a 
modified,  fused  blastopore  (p.  39),  their  origin  is  fundamentally  like  that  in  Amphioxus 
and  amphibia.  From  its  external  position  and  developmental  relations  the  parent  blasto- 
poric  tissue  is  often  styled  ectoderm;  especially  in  embryos  with  a primitive  streak  this  is 
convenient  and  unobjectionable.  It  will  be  evident,  therefore,  that  although  the  ultimate 
source  of  both  mesoderm  and  notochord  is  from  an  indifferent  ‘ectoderm,’  the  notochord, 
once  formed,  is  true  mesoderm. 

The  Notochord  or  Chorda  Dorsalis. — As  the  primitive  streak  recedes  caudad  during 
development,  the  head  process  is  progressively  lengthened  at  its  expense.  Ultimately,  the 
primitive  streak  becomes  restricted  to  the  tail  region  and  serves  as  a growth  zone  there, 
whereas  the  entire  remainder  of  the  body  is  built  around  the  head  process  as  an  axis.  The 
original  position  of  the  primitive  knot  corresponds  to  the  junction  of  head  and  neck  in  the 
future  body.  In  later  stages,  the  rod-like  notochord  extends  from  head  to  tail  in  the  mid- 
plane (Fig.  91).  It  becomes  enclosed  in  the  centra  of  the  vertebrae  and  in  the  base  of  the 
cranium,  and  eventually  degenerates.  In  Amphioxus,  the  notochord  forms  the  only  axial 
skeleton,  and  it  is  persistent  in  the  vertebrae  of  fishes  and  amphibians.  In  adult  man, 
traces  are  found  as  ‘pulpy  nuclei’  in  the  intervertebral  discs. 

Twinning. — LTsually  but  one  human  ovum  is  produced  and  fertilized  at  coitus.  The 
simultaneous  development  of  two  or  more  embryos  is  due  commonly  to  the  ripening, 
expulsion,  and  subsequent  fertilization  of  an  equal  number  of  ova.  In  such  cases  ordinary, 
or  fraternal  livins,  triplets,  and  so  on,  of  the  same  or  opposite  sex  result;  properly  speaking, 
they  are  not  twins  at  all.  Identical,  or  duplicate  twins,  that  is,  those  true  twins  always  of 
the  same  sex  and  strikingly  similar  in  form  and  feature,  arise  from  two  growing  points  on 
the  embryonic  cell  mass,  each  of  which  develops  as  a separate  embryo  within  the  common 
chorion.  The  identical  quadruplets  of  certain  armadillos  are  known  to  result  from  the 
division  of  a single  blastoderm  into  four  parts.  Separate  development  of  the  cleavage 
cells  can  also  be  produced  experimentally  in  many  of  the  lower  animals. 

Occasionally  twins  are  conjoined.  All  degrees  of  union,  from  almost  complete  sep- 
aration to  fusion  throughout  the  entire  body-length,  are  known.  If  there  is  considerable 
disparity  in  size,  the  smaller  is  termed  the  parasite;  in  such  cases  the  extent  of  attachment 
and  dependency  grades  down  to  included  twin  (fetus  in  fetu)  and  tumor-like  fetal  inclu- 


ORIGIN  OF  THE  MESODERM,  NOTOCHORD  AND  NEURAL  TUBE 


43 


sions.  In  some  ‘monsters’  the  duplication  is  partial,  as  doubling  of  the  head  or  legs.  All 
of  these  terata,  like  identical  twins,  are  the  products  of  a single  ovum,  but  variably  fused  in 
accordance  with  their  original  degree  of  separation  on  the  blastodermic  mass. 

Stockard  reduces  the  primary  cause  of  all  non-hereditary  abnormal  developments, 
including  twins,  to  a single  factor — developmental  inhibition  or  arrest;  the  exact  type  of 
deformity  that  results  depends  solely  on  the  precise  moment  when  the  interruption  occurs. 
A slowing  of  the  developmental  rate  at  the  critical  moment  (gastrulation)  when  one  of  sev- 
eral potential  embryonic  axes  is  about  to  assert  its  dominance,  causes  it  to  lose  its  original 
advantage  and  one  or  more  neighboring  points  may  then  appear  as  additional  axes.  The 
direct  cause  of  the  arrest  is  referred  to  retarded  oxidations. 


CHAPTER  III 


IMPLANTATION  AND  FETAL  MEMBRANES 

The  conditions  under  which  vertebrate  eggs  develop  vary  markedly. 
In  all  vcrtel)rates  below  mammals  the  eggs  are  laid  and  develop  in  the 
surrounding  medium,  aided  sometimes  (especially  in  reptiles  and  birds) 
by  parental  protection  and  incubation.  As  a group,  the  mammals  alone 
develo]:>  their  young  within  the  genital  tract  of  the  mother. 

The  embryos  of  fishes  and  amphibia  grow  rapidly  to  immature  forms 
capable  of  independent  existence.  All  other  vertebrates  are  much  farther 
advanced  at  Ihrth  and  accordingly  form  various  organs,  of  use  during 
develoi)ment  only.  Especially  in  higher  mammals  has  the  absence  of 
yolk,  and  the  resulting  physiological  dependence  upon  the  mother,  led  to 
the  greatest  elaboration  of  these  ajopendages. 

vSuch  fetal  organs  include  the  yolk  sac  and  stalk,  the  allantois,  amnion, 
and  chorion.  They  have  to  do  with  the  nutrition  and  respiration  of  the 
embryo,  and  the  elimination  of  katabolic  wastes.  In  higher  mammals,  the 
chorion  is  associated  intimately  with  the  uterine  mucosa  and  forms  with 
it  an  important  organ  called  the  placenta. 

THE  FETAL  MEMBRANES  OF  REPTILES  AND  BIRDS 
Development  is  similar  in  both  classes.  The  chick  illustrates  typi- 
cally the  manner  of  membrane  formation. 

Amnion  and  Chorion.  —The  embryo  develops  in  the  center  of  the 
blastoderm,  which  first  lies  like  a disc  upon  the  massive  yolk  (Fig.  6). 
Later,  the  periphery  of  the  blastoderm,  not  concerned  in  embryo  formation, 
expands  and  encloses  the  yolk  mass.  This  envelope  consists  of  somato- 
pleure  and  splanchnopleure,  separated  by  the  coelom  (Fig.  4).  The  amnion 
and  chorion  arise  from  the  somatopleure.  This  double  layer  (ectoderm  and 
somatic  mesoderm)  is  thrown  up  into  crescentic  folds,  just  in  front  of  and 
behind  the  embryo  (Fig.  36  A).  Gradually,  the  hood-like  folds  close  in  from 
all  sides  until  they  meet  and  fuse  over  the  embryo  ( Fig.  36  B-C) . The  inner 
somatopleuric  layer,  thus  formed,  is  the  amnion;  it  constitutes  a protective 
sac,  lined  with  ectoderm  and  soon  filled  with  fluid,  within  which  the 
embryo  is  suspended.  The  outer  of  the  two  somatopleuric  sheets  is  the 
chorion.  It  lies  next  the  shell  and  is  separated  by  the  extra-embryonic 
coelom  from  the  enclosed  embryo  and  its  other  membranes  (Fig.  37). 

Yolk  Sac. — As  the  embryo  enlarges,  its  original  connection  with  the 
extra-embryonic  blastoderm  becomes  a slender  stalk,  uniting  embryo  and 


44 


THE  FETAL  MEMBRANES  OF  REPTILES  AND  BIRDS 


45 


yolk  (Fig.  37).  It  is  designated  the  yolk  stalk,  whereas  the  yolk,  enveloped 
by  extra-embryonic  blastoderm,  is  the  yolk  sac.  Vitelline  blood  vessels 
ramify  on  the  surface  of  the  yolk  sac  and  through  them  all  the  food 


.4 


B 


Fig.  36. — Diagrams  in  a sagittal  plane  illustrating  the  development  of  the  fetal  membranes 
of  most  amniotes  (after  Gegenbaur  in  McMurrich).  Ectoderm,  mesoderm,  and  entoderm  are 
represented  by  heavy,  light,  and  dotted  lines  respectively.  .1/.,  Allantois:  Am.,  amniotic 
cavity;  I5.,  j^olk  sac. 


/ 


Fig.  37. — Diagram  of  a five-day  chick  embryo  and  its  membranes  (Marshall).  X 1.5. 


Allantois  Embryo  Amnion 


Yolk  sac 


Chorion 
Shell 


Air  chamber 


Shell  membrane 

Margin  of  area  vascitlosa 


material  of  the  liquefied  yolk  is  conveyed  to  the  chick  during  the  incuba- 
tion period. 


46 


IMPLANTATION  AND  FETAL  MEMBRANES 


Allantois. — There  is  an  early  outpouching  of  the  ventral  floor  of  the 
gut,  near  its  hind  end.  This  entodermal  diverticulum  pushes  outward  into 
the  extra-embryonic  coelom,  carrying  before  it  an  investment  of  splanchnic 
mesoderm  (Fig.  36).  It  forms  a vesicle,  known  as  the  allantois,  which 
develops  rapidly  into  a large  sac,  connected  to  the  hind-gut  by  the  narrower 
allantoic  stalk  (Fig.  37).  Finally,  the  allantois  flattens  and  fuses  with  the 
chorion,  just  underlying  the  porous  shell  (Fig.  36  D).  The  blood  vessels 
that  ramify  in  the  combined  mesodermal  wall  are  situated  favorably  for 
gaseous  interchange,  and  the  allantois  becomes  the  embryonic  respiratory 
organ.  The  allantoic  cavity  also  serves  in  its  primitive  capacity  as  a 
reservoir  for  the  excreta  of  the  embryonic  kidneys,  and  the  wall  assists  in 
the  absorption_of  albumen. 

THE  FETAL  MEMBRANES  OF  MAMMALS 

Amnion  and  Chorion. — In  most  mammals  these  membranes  arise  by 
folding,  as  in  reptiles  and  birds.  Some  (guinea  pig;  hedgehog;  bat;  pri- 
mates) form  an  amnion  precociously  in  an  entirely  different  manner. 
In  the  bat,  fluid-filled  clefts  appear  in  the  interior  of  the  embryonic  cell 
mass;  these  coalesce  and  constitute  the  amnion  cavity  (Fig.  38).  Later, 
a layer  of  somatic  mesoderm  envelops  its  ectodermal  roof  and  the  structural 
outcome  is  identical  with  the  type  derived  by  folding.  The  deer  and  sheep 
show  a method  transitional  between  these  extremes;  the  embryonic  mass 
hollows  and  its  roof  ruptures;  then  the  definitive  amnion  develops  by  fold- 
ing. The  same  group  that  derives  an  amnion  by  dehiscence,  forms  a 
chorion  from  the  outer  trophectoderm  layer  of  the  blastodermic  vesicle, 
to  which  somatic  mesoderm  is  added  (Fig.  40). 

Yolk  Sac. — The  yolk  sac  of  monotremes  resembles  that  of  birds,  but  in 
higher  forms  an  actual  yolk  mass  is  lacking.  There  are  numerous  early 
developmental  variations.  In  the  majority,  the  yolk-sac  entoderm  spreads 
beneath  the  trophectoderm  shell  and  for  a time  lines  it  (Fig.  38) ; when  the 
extra-embryonic  mesoderm  and  coelom  appear,  the  entoderm  becomes 
clothed  with  the  splanchnic  layer  and  the  sac  is  reduced  in  relative  size. 
On  the  contrary,  the  yolk  sac  of  primates  is  small  from  the  first  and  remains 
as  a diminutive  central  vesicle  (Figs.  25  and  40).  In  rodents,  carnivores, 
and  split-hoofed  mammals  it  early  attains  a large  size,  but  ceases  growth  as 
the  allantois  comes  to  prominence.  The  splanchnic  mesoderm  of  all 
groups  bears  the  vitelline  blood  vessels.  Many  animals  with  a highly 
developed  yolk  sac  effect  an  intimate  association  (through  union  with  the 
chorion)  with  the  uterine  mucosa.  There  is  thus  formed  a transitory 
yolk-sac  placenta.  In  some  marsupials  and  insectivores  this  relation 
persists. 


THE  FETAL  MEALBRANES  OF  MAMMALS 


47 


Allantois. — Alany  mammals,  like  reptiles  and  birds,  form  an  allantois 
by  the  sacculation  of  gut-splanchnopleure  into  the  extra-embryonic 
coelom.  In  some  it  remains  small,  and,  especially  in  certain  marsupials, 
does  not  come  in  contact  with  the  chorion.  On  the  contrary,  in  carnivores 
and  ungulates  it  becomes  very  large  and  lines  the  chorionic  sac  (Fig.  39). 
A goat  embryo  of  two  inches  has  an  allantois  two  feet  long. 


Inner  cell  mass 


Inner  cell  mass  Trophectoderm 


Embryonic  ectoderm 


Entoderm 


Fig.  38. — Stages  of  amnion  formation  in  the  bat  (Van  Beneden).  X about  160. 

Primates  have  a tiny,  tubular  allantois;  it  grows  into  and  lies  within 
the  body  stalk,  which  is  a bridge  of  mesoderm  connecting  the  embryo  to  he 
chorion  (Fig.  40  D).  Allantoic,  or  umbilical  blood  vessels  accompany  he 
allantois. 

The  Placenta. — The  egg-laying  monotremes  develop  under  the  same 
nutritive  and  respiratory  conditions  as  do  reptiles  and  birds.  The  mar- 
supials, after  a brief  gestation  period,  give  birth  to  immature  }mung;  their 


48 


IMPLANTATION  AND  FETAL  MEMBRANES 


chorion,  therefore,  remains  as  a smooth  membrane  but  in  close  apposition 
with  the  vascular  uterine  mucosa.  The  yolk  sac  is  large  and  in  some  forms 
it  unites  with  the  chorion,  apparently  to  serve  as  a nutritive  path  from 
uterus  to  embryo. 

In  all  higher  mammals,  the  chorion  is  beset  with  vascular  villi  and 
there  is  a more  or  less  intimate  relation,  which  persists  throughout  gestation, 
between  the  uterine  mucosa  and  the  chorionic  vesicle.  This  arrangement 
results  in  the  formation  of  an  organ,  the  placenta,  specialized  for  the  nutri- 
tion of  the  embryo  and  for  its  respiration  and  excretion. 


Entoderm  of  primitive  gut  Amnion 


Fig.  39. — Diagram  of  the  fetal  membranes  and  allantoic  placenta  of  a pig  embryo,  in  median 
sagittal  section  (adapted  by  Prentiss). 

The  form  and  extent  of  the  placenta  vary  in  accordance  with  the 
final  distribution  of  the  chorionic  villi.  The  pig  and  horse  have  villi 
diffusely  scattered  over  the  entire  chorion.  In  ruminants,  they  occur  in 
broadly  scattered  tufts,  interspaced  with  smooth  stretches  of  chorion.  The 
villi  of  carnivores  constitute  a girdle-like  band  about  the  chorionic  sac. 
In  rodents,  insectivores,  bats,  and  primates,  the  villi  are  limited  to  a patch- 
like disc  (Fig.  48). 

There  is  likewise  a structural  series,  based  on  the  degree  of  fetal- 
maternal  intimacy.  At  the  bottom  of  the  scale  stands  the  mere  apposi- 
tion of  the  uterine  mucosa  and  the  avillous  chorion  of  marsupials. 


THE  FETAL  MEMBRANES  OF  MAN 


49 


Simplest  of  the  forms  with  chorionic  villi  is  the  condition  illustrated 
by  the  pig  or  horse  (Fig.  39).  The  allantois,  developing  as  in  the  chick, 
comes  in  contact  and  fuses  with  the  chorion.  Allantoic  vessels  lie  in  the 
combined  mesoderm.  Meanwhile,  the  external  ectoderm  of  the  amnion 
has  closely  applied  itself  to  the  uterine  epithelium,  and,  when  the  chorionic 
villi  appear,  they  fit  into  corresponding  pits  in  the  mucosa.  Nutritive 
substances  and  oxygen  from  the  maternal  blood  must  pass  through  both 
layers  of  epithelium  before  entering  the  allantoic  vessels.  In  the  same 
manner,  waste  products  from  the  embryo  pass  in  the  reverse  direction. 
The  allantois  has  therefore  become  important,  not  only  as  an  organ  of 
respiration  and  excretion,  as  in  reptiles  and  birds,  but  also  as  an  organ 
of  nutrition.  Through  its  vessels  it  has  taken  on  the  function  belonging 
to  the  yolk  sac  of  lower  vertebrates,  and  the  rudimentary,  yolkless  sac  of 
higher  mammals  is  now  explained. 

This  general  scheme  of  the  ungulates  is  modified  by  an  advance 
among  its  ruminant  subgroup.  Here  the  villi  penetrate  deeper  and  come 
in  closer  relation  with  the  connective  tissue  about  the  maternal  vessels 
by  a partial  destruction  of  the  uterine  epithelium.  At  the  end  of  gestation 
the  chorionic  villi  of  the  pig,  horse,  and  ruminant  are  merely  withdrawn, 
and  the  maternal  mucosa  is  not  lost. 

In  carnivores  there  is  marked  destruction  of  the  mucosa,  so  that  the 
chorionic  epithelium  about  the  maternal  vessels  is  separated  from  the 
circulating  blood  by  endothelium  alone. 

The  highest  type,  as  in  rodents  and  primates,  is  characterized  by  a 
superficial  erosion  and  destruction  of  the  uterine  mucosa,  so  that  the 
chorionic  villi,  dangling  in  cavernous  spaces,  are  bathed  by  the  maternal 
blood  which  issues  from  eroded  vessels  (Fig.  34).  In  this  and  the  preced- 
ing type,  the  changes  are  so  profound  and  the  fusions  so  intimate  that  the 
mucosa  is  largely  sloughed  at  birth  as  a decidua.  The  chorion  was 
important  in  the  ungulate  chiefly  as  it  brought  the  allantois  into  close 
relation  with  the  uterine  wall,  but  in  man  and  most  unguiculates  it 
assumes  the  several  placental  functions,  and  the  allantois,  now  superseded 
like  the  yolk  sac,  in  turn  becomes  rudimentary. 

THE  FETAL  MEMBRANES  OF  MAN 

Amnion. — -In  the  youngest  known  human  embryo  (Miller)  the 
embryonic  mass  is  solid  (Fig.  40  A),  but  its  ectoderm  indicates  a stage 
preparatory  to  the  formation  of  amnion  clefts.  As  an  amniotic  cavity 
is  present  in  the  slightly  older  embryo  described  by  Bryce-Teacher 
(Fig.  40  B),  the  method  of  origin  must  be  by  direct  splitting  as  in  the  bat 
(p.  46).  These  specimens  likewise  lack  a coelom  in  the  precociously 
formed  extra-embryonic  mesoderm,  whereas  all  older  embryos  possess 
somatic  and  splanchnic  layers  bounding  a more  or  less  extensive  coelomic 


5° 


IMPLANTATION  AND  FETAL  MEMBRANES 


cleft  (Fig.  40  C).  Somatic  mesoderm  then  covers  the  primitive  ecto- 
dermal roof  of  the  amniotic  cavity  (Fig.  40  D) ; this  order  of  layering 
is  identical  in  all  amniotes. 

At  first  there  is  a broad  union  between  the  amnion  and  the  external 
shell  of  trophectoderm  (Fig.  40  C),  but  this  becomes  reduced  by  the 
continued  extension  of  the  coelomic  cavity  until  presently  it  is  limited 
to  the  caudal  end  of  the  embryo  alone  (Fig.  40  D).  This  narrow,  meso- 


A 


B 


Ectoderm 

Amniotic  cavity 

Coelom 
Trophectoderm 
A 

Entoderm 
Mesoderm 


C 


D 


Ectoderm  of  embryo 


Ectoderm  of  amnion 


Allantois 


mniotic  cavity 

Trophectoderm 
Yolk  sac 
Entoderm 


Ectoderm  of 

Cavity  of  amnion 
Mesoderm  of 
amnion 
Ectoderm  of 
chorion 
Cavity  of  yolk 


Splanchnic  Entoderm 
mesoderm 

onic  coelom  gf  ygip  ^ac 

Extra-embry- 
, . onic  coelom 

horionic  meso- 
derm 

T ro phoderm  of  chorion 


Body  stalk 


Chorionic  villi 


Fig.  40. — Diagrams  of  early  human  embryos  (adapted  by  Prentiss).  A,  Miller  (modified); 
B,  Bryce-Teacher  (modified);  C,  Peters;  D,  Spee. 


dermal  bridge,  into  which  the  allantois  and  its  vessels  grow,  is  the  body 
stalk  (Fig.  43). 

Hence,  from  the  first,  the  human  amniotic  cavity  is  closed.  The 
base  of  the  amnion  is  attached  to  the  periphery  of  the  embryonic  disc, 
which  also  constitutes  the  floor  of  the  cavity  (Fig.  32  A).  The  amnion 
becomes  a thin,  pellucid,  non-vascular  membrane,  lined  with  a simple 
epithelium  (Figs.  43,  61  and  65).  The  amniotic  cavity  enlarges  rapidly  at 
the  expense  of  the  extra-embryonic  coelom,  and,  at  the  end  of  the  second 


THE  FETAL  MEiMBRANES  OF  MAN 


51 


month,  fills  the  chorionic  sac  (Fig.  51).  It  then  attaches  loosely  to  the 
chorionic  wall,  thereby  obliterating  the  extra-embryonic  body  cavity 
(Fig.  50). 

Amniotic  fluid  fills  the  sac.  Its  immediate  origin  (fetal  or  maternal) 
is  disputed.  During  the  early  months  of  pregnancy  the  embryo  is  sus- 
pended by  the  umbilical  cord  in  this  fluid  (Fig.  51).  Throughout  gesta- 
tion the  amniotic  fluid  serves  as  a protective  water  cushion,  equalizing 
pressures  and  preventing  adherence  of  the  amnion.  At  parturition,  it 
acts  as  a fluid  wedge  to  dilate  the  uterine  cervix.  The  embryo  is  protected 
from  maceration  by  a fatty  skin-secretion,  the  vernix  caseosa. 

During  the  early  stages  of  childbirth  the  membranes  usually  rupture, 
and  about  a liter  of  amniotic  fluid  escapes  as  the  ‘waters.’  If  the  tough 
amnion  fails  to  burst,  the  head  is  delivered  enveloped  in  it,  and  it  is  then 
popularly  known  as  the  ‘caul.’ 

Anomalies. — When  the  amniotic  fluid  is  excessive  in  volume,  the  condition  is  designated 
' hydramuios.’  If  less  than  the  optimal  amount  is  present,  the  amnion  may  adhere  to  the 
embryo  and  cause  malformations.  Fibrous  bands  sometimes  extend  across  the  amnion 
cavity.  As  pressure  increases  during  growth,  they  may  cause  scars  and  the  splitting  or 
even  amputation  of  parts. 


Fig.  41. — Section  of  Peters’  0.19  mm.  human  embryo  (about  fifteen  da}^s).  The  portion  of 
extra-embryonic  ccelom  shown  is  limited  below  by  a strand  of  the  magma  reticulare. 

Yolk  Sac. — The  entodermal  portion  of  the  Miller  embryo  is  solid 
(Fig.  40  A),  but  in  all  other  early  specimens  it  forms  a small  vesicle,  lined 
with  a single  layer  of  entoderm  and  covered  with  splanchnic  mesoderm 
(Figs.  40  B-D,  41  and  43).  In  embryos  of  1.5  to  2.0  mm.,  the  entodermal 
roof  of  this  vesicle  begins  to  form  the  fore-  and  hind-gut  which  are  then 
connected  by  a slightly  narrowed  region  to  the  yolk  sac  proper  (Figs.  43 


52 


IMPLANTATION  AND  FETAL  MEMBRANES 


and  44).  With  the  growth  of  the  head-  and  tail  regions  of  the  embryo 
there  is  an  apparent  progressive  constriction  of  the  yolk  sac  (Figs.  60,  61 
and  64).  This,  however,  is  a deception.  Both  embryo  and  yolk  sac 
enlarge,  whereas  the  region  of  union  lags  in  transverse  development  but 
elongates  into  the  slender  yolk  stalk  (Fig.  42). 

The  yolk  stalk  becomes  incorporated  in  the  umbilical  cord  (Figs. 
45,  64  and  65).  It  loses  its  attachment  with  the  gut  in  embryos  of  7 
mm.  and  soon  degenerates.  Even  earlier,  the  yolk  sac  has  attained  its 
final  diameter  of  about  i cm. ; it  persists  and  may  be  found  at  birth  adherent 
to  the  amnion  in  the  placental  region  (Fig.  55).  The  yolk  sac  of  man  is 
a vestige  containing  a coagulum  but  no  yolk  (Fig.  41).  Blood  vessels 
arise  very  early  in  its  mesoderm  (Figs.  43  and  44)  and  institute  a vitelline 
circulation  with  the  embryo. 


/ 


Fig.  42. — Yolk  sac  and  .stalk  of  a 20  mm.  human  embryo  (Prentiss).  X ii. 

Anomalies.  If  that  portion  of  the  yolk  stalk  between  the  intestine  and  umbilicus 
remains  pervious  it  constitutes  a fecal  fistula  through  which  intestinal  contents  may  escape. 

In  2 per  cent  of  all  adults  there  is  a persistence  of  the  proximal  end  of  the  yolk  stalk, 
to  form  a pouch,  Meckel’s  diverticulum  of  the  ileum.  This  varies  between  3 and  9 or  more 
cm.  in  length  and  lies  about  80  cm.  above  the  colic  valve.  The  divei'ticulum  is  important 
surgically  as  it  sometimes  telescopes  into  the  intestinal  lumen  and  occludes  it. 

Allantois.  —Although  the  allantois  is  absent  in  the  youngest  embryos 
known,  it  nevertheless  appears  very  early — even  before  the  gut.  In  the 
Spee  specimen,  the  allantois  is  a slender  tube  extending  into  the  meso- 
derm of  the  body  stalk  (Fig.  43).  It  never  becomes  saccular,  as  in  most 
lower  amniotes.  Since  the  human  allantois  arises  so  precociously,  it 
does  not  develop  as  an  evagination  of  the  hind-gut  into  the  extra-embry- 
onic coelom;  yet  the  body  stalk,  which  contains  the  allantois,  represents 
mesoderm  into  which  the  coelom  has  failed  to  penetrate. 

Elongation  extends  the  allantoic  tube  as  far  as  the  chorion  (Figs. 
44,  71  and  184),  and,  when  the  developing  umbilical  cord  includes  the 
allantois  as  a component,  it  at  first  is  as  long  as  the  cord  (Figs.  45  and  51). 
Soon,  however,  growth  ceases  and  at  birth  the  only  remnant  is  a tenuous, 
and  generally  discontinuous,  solid  strand. 


THE  FETAL  MEMBRANES  OF  MAN 


53 


Amnion 
Embryonic  disc 

A nlage  of  heart 


Splanchnic  mesoderm 


Body  stalk 
Primitive  streak 


Allantois 


I oik  sac 


Entoderm 


Somatic  mesoderm 


Blood  vessel 


Fig.  43. — Sagittal  section  of  Spec’s  1.54  mm.  human  embryo. 


Villi  of  chorion 


Chorion 


Fig.  44. — Mali’s  2.0  mm.  human  embryo  in  median  sagittal  section  (adapted  by  Prentiss). 

X 23. 


54 


IMPLANTATION  AND  FETAL  ^MEMBRANES 


Umliilical  blood  vessels  accompany  the  allantois;  these  also  reach 
the  chorion  and  vascularize  it  (Figs.  51  and  184).  When  the  chorion 
becomes  a part  of  the  placenta  it  performs  all  the  functions  of  nutrition, 
res]:iiration,  and  excretion.  Like  the  yolk  sac,  the  allantois  is  a super- 
seded rudiment. 

Chorion.  -The  human  chorion  is  derived  directly  from  the  trophecto- 
derm  layer  of  the  blastodermic  vesicle  to  which  is  added  extra-embryonic 
mesoderm  (Fig.  40).  The  trophectoderm  of  the  youngest  known  embryos 
has  already  given  rise  to  an  outer  syncytial  layer,  the  irophodcrm,  but 
the  mesoderm  is  solid.  In  slightly  older  specimens,  the  mesoderm  is 
cleft  by  the  extra-embryonic  cmlom  and  its  outer,  or  somatic,  layer  lines 
the  chorion  (Figs.  40  B-I)  and  46).  The  chorion  forms  villous  processess 
(Fig.  48).  At  first  these  are  solid  ectoderm,  the  primary  villi  (Fig.  40  C), 
but  soon  the  chorionic  mesoderm  invades  them  as  central  cores  (Fig.  43) 
and  allantoic,  or  umbilical  blood  vessels  ramify  in  their  branches.  Such 
villi  are  secondary,  or  true  villi  (Figs.  51  and  65).  The  further  history 
of  the  chorion  is  inseparable  from  placental  development  (p.  62). 

Umbilical  Cord. — As  the  embryo  enlarges,  its  ventral,  unclosed 
area,  bounded  by  the  edge  of  the  amnion,  becomes  relatively  smaller 
(Fig.  45  A,  5).  For  a time  the  amnion  attaches  close  to  the  embryo, 
but,  during  the  sixth  week,  growth  of  the  adjoining  body  wall,  accompanied 
by  an  elongation  of  the  body  stalk,  causes  the  amnion  to  recede  from  the 
iimhiliciis.  The  tubular  structure  thus  formed  is  the  itmhilical  cord 
(Fig.  45  C).  It  encloses  both  yolk  stalk  and  allantois,  and  includes  a 
portion  of  the  coelom.  Henceforth,  the  umbilical  cord  connects  the 
embryo  to  that  part  of  the  chorion  which  constitutes  the  fetal  half  of  the 
placenta  (Figs.  51  and  55).  The  umbilical  cord  is  actually  an  embryonic 
growth,  and  the  amnion  merely  attaches  to  its  distal  end  ( Fig.  65). 

The  cord  is  covered  with  ectodermal  epithelium  and  contains, 
embedded  in  mucous  tissue  (jelly  of  Wharton):  (i)  the  yolk  stalk  (and 
in  early  stages  its  vitelline  vessels);  (2)  the  allantois;  (3)  the  allantoic  or 
umbilical  vessels  (two  arteries  and  a single,  large  vein ) . The  mucous  tissue, 
peculiar  to  the  umbilical  cord,  comes  from  mesenchyme;  it  bears  neither 
capillaries  nor  nerves.  Between  the  sixth  and  tenth  weeks,  the  gut 
extends  into  the  coelom  of  the  cord  and  forms  a temporary  umbilical 
hernia  there  (Fig.  96).  After  it  is  withdrawn,  the  cavity  of  the  cord 
disappears. 

The  mature  cord  is  about  1 .5  cm.  in  diameter  and  attains  an  average 
length  of  50  cm.  Its  insertion  is  usually  near  the  center  of  the  placenta 
(Fig.  56),  but  may  be  marginal  or  even  on  the  adjoining  membranes. 
A spiral  twist  appears  (Fig.  53),  just  how  is  not  known,  and  the  blood 
vessels  sometimes  curl  in  masses  which  cause  external  bulgings,  designated 


THE  FETAL  MEMBRANES  OF  MAN 


55 


Chorionic  villus 
i oik  sac 


Amnion 


Chorion  IcEve 


Allantois 


Fig.  45. — Diagrams  of  the  development  of  the  human  umbilical  cord  (DeLee).  a.c.,  Amniotic 

cavity;  exc.,  extra-embryonic  coelom. 


IMPLANTATION  AND  PETAL  MEIVLBRANES 


56 

‘false  knots.’  True  knots  are  known  also.  The  cord  may  wind  about 
the  neck  or  extremities  of  a fetus  and  induce  atrophy  or  even  amputation. 

Implantation  and  Early  Mucosal  Relations 
During  the  events  of  cleavage  and  the  formation  of  a morula  and 
blastodermic  vesicle,  the  ciliated  lining  of  the  uterine  tube  steadily  trans- 
ports the  ovum  downward.  Early  in  this  period  of  migration  and  develop- 
ment, the  ovum  loses  its  corona  radiata  cells  and  pellucid  membrane. 
In  about  eight  days  it  probably  reaches  the  uterus,  having  attained  a 
stage  something  like  Fig.  40  A,  although  the  vesicle  is  only  about  0.2 
mm.  in  diameter.  It  is  evident,  therefore,  that  the  foregoing  sections  of 
this  cha])ter  describe  changes  which  occur  largely  after  implantation, 
rather  than  before  it. 


Maternal  vessel  Traj>lwderm  Uterine  gland  Troplioderm 


Fig.  46. — Section  through  a human  embryo  of  0.19  mm.,  embedded  in  the  uterine  mucosa 
(semidiagrammatic  after  Peters),  am.,  Amniotic  cavity;  h.s.,  body  stalk;  eel.,  ectoderm  of 
embryo;  ent.,  entoderm;  ?nes.,  mesoderm;  y.s.,  yolk  sac. 

Implantation  comprises  the  process  by  which  the  embryonic  vesicle 
becomes  embedded  in  the  uterine  mucosa.  Actual  observations  on  the 
human  ovum  are  lacking,  but  from  careful  studies  on  the  earliest  speci- 
mens, and  from  more  complete  observations  on  other  mammals,  the  course 
of  events  is  reasonably  certain. 

The  ovum  penetrates  the  mucosa  as  would  a parasite,  the  tropho- 
derm  supposedly  producing  an  enzym.e  which  digests  away  the  maternal 


IMPLANTATION  AND  EARLY  MUCOSAL  RELATIONS 


57 


tissues  until  the  embryo  is  entirely  embedded.  The  Peters  specimen, 
shown  in  Fig.  46,  is  well  established  and  the  chorionic  vesicle  has  an 
internal  diameter  of  more  than  a millimeter.  Its  point  of  entrance  is 
marked  by  the  customary  fibrin  clot  which  soon  disappears,  and  the 
defect  is  repaired. 

Continued  rapid  growth  of  the  embryo  necessitates  a correspondingly 
progressive  erosion  of  the  maternal  tissues.  This  causes  extravasations 
of  blood  which  collect  in  large  vacuoles  in  the  invading  trophoderm  and 
form  blood  lacunae  (Fig.  46).  The  lacunce  break  up  the  trophoderm  into 
solid  cords,  composed  of  both  the  inner  cellular  and  outer  syncytial  layers. 
These  constitute  the  primary  villi.  It  is  the  syncytial  layer  that  is  active 
in  the  destruction  of  the  uterine  tissues,  and  probably  also  in  the  absorp- 
tion of  blood  and  tissue  products  (emhryotroph)  for  the  early  nutrition  of 
the  embryo. 

Next,  there  are  changes  leading  to  the  definitive  [hemotrophic)  type 
of  nutrition.  Chorionic  mesoderm  extends  into  the  primary  villi,  and 


Canalized  fibrin  Maternai  capillary  Inlennllous  space 
Fig.  47. — Diagram  of  the  early  development  of  chorionic  villi  and  placenta  (after  Peters). 


Syncytial. 

trophoderm^ 


M esoderm 


Core  of  villus 


Canalized  fibrin 


Endothelium 


branching  secondary  or  true  villi  result  (Figs.  43  and  47).  During  the 
development  of  villi  the  blood  lacunas  in  the  original  trophoderm  shell 
expand,  run  together,  and  produce  intervillous  spaces  which  surround 
the  villi  and  bathe  their  epithelium  (Fig.  47).  The  formerly  .spongy 
trophoderm  is  now  reduced  to  a continuous  layer  covering  the  outer 
surfaces  of  the  villi  and  chorion.  Branches  of  the  umbilical  vessels 
develop  in  the  mesoderm  of  the  chorion  and  villi  (Fig.  51).  The  meso- 
dermal core  of  each  villus  and  its  branches  is  then  covered  by  a 
two-layered  epithelium;  an  inner,  ectodermal  layer  (of  Langhans)  with 
distinctly  outlined  cuboidal  cells,  and  an  outer,  syncytial  trophoderm  layer 
(Figs.  47  and  53  A).  The  epithelium  also  forms  solid  columns  of  cells 
which  anchor  the  ends  of  certain  villi  to  the  uterine  wall  (Fig.  47). 


58 


IMPLANTATION  AND  FETAL  MEMBRANES 


In  the  vessels  of  the  chorionic  villi,  the  chorionic  circulation  of  the 
embryo  is  established.  The  blood  vessels  of  the  uterus  open  into  the 
intervillous  blood  s]iaces,  and  here  the  maternal  blood  circulates  and 
bathes  the  syncytial  trophoderm  of  the  villi  (Figs.  47  and  54).  The 
transfer  of  nutritive  substances  and  oxygen  to  the  fetal  blood  takes  place 
through  the  walls  of  the  chorionic  villi,  whereas  fetal  wastes  pass  in  the 
reverse  direction.  The  tro])hoderm,  like  endothelium,  prevents  the 
coagulation  of  maternal  blood.  According  to  Mall,  it  also  forms  a wall 
which  dams  or  iilugs  the  maternal  blood  vessels  as  soon  as  eroded,  and, 
with  the  decidua  (p.  62),  limits  the  flow  of  blood  into  the  intervillous 
spaces. 

\dlli  at  first  cover  the  entire  surface  of  the  chorion  (Fig.  40  D).  As 
the  embryo  enlarges,  the  villi  next  the  uterine  cavity  become  both  com- 
pres.sed  and  remote  from  the  Idood  supply  (Fig.  51).  During  the  fourth 
week  these  villi  atrophy  and  disappear  (Fig.  48).  This  leaves  a smooth 


B 

Fig.  48. — Human  chorionic  vesicles  of  five  and  seven  weeks  (De  Lee).  The  chorion  laeve  and 
chorion  frondosum  are  apparent.  Natural  size. 

surface,  called  the  chorion  Iceve.  The  villi  adjacent  to  the  uterine  wall 
persist  as  the  chorion  frondosum  and  become  the  fetal  part  of  the  placenta 
(Fig.  49). 

The  Decidual  Membranes 

Two  sets  of  important  changes  take  place  normally  in  the  uterine 
mucosa.  One  of  these  is  periodic,  between  puberty  and  the  menopause, 
and  is  the  cause  of  menstruation.  It  is  comparable  to  the  oestrus  cycle  in 
lower  animals,  and  may  also  be  regarded  as  preparatory  to  the  second  set  of 
changes  which  appear  only  in  pregnancy  and  give  rise  to  the  decidual 
membranes  and  placenta. 


THE  DECIDUAL  MEMBRANES 


59 


Menstruation. — The  periodic  changes  that  accompany  the  phenome- 
non of  menstruation  form  a cycle  which  occupies  twenty-eight  days. 
This  period  is  divisible  into  four  phases ; 

I.  Tumefaction  (six  days).  The  uterine  mucosa  thickens  both 
because  of  vascular  congestion  and  cellular  multiplication.  Blood  escapes 
from  the  enlarged  capillaries  and  forms  subepithelial  masses.  The  uterine 
glands  elongate  and  their  deeper  portions  especially  are  convoluted  and 


dilated  with  secretion.  The  mucosa  thus  shows  a superficial,  compact 
layer  and  a deep,  spongy  layer. 

2.  Menstruation  proper  (four  days).  The  superficial  blood  vessels 
rupture  and  add  to  the  blood  and  glandular  discharge  which  is  escaping  into 
the  uterine  cavity.  The  surface  epithelium  and  a portion  of  the  under- 
lying tissue  may  or  may  not  be  desquamated. 

3.  Restoration  (five  days).  The  vascular  engorgement  disappears. 
Extravasated  blood  corpuscles  are  resorbed  or  cast  off.  The  epithelium, 
glands,  and  capillaries  are  repaired. 


6o 


IMPLANTATIOX  AND  FETAL  MEMBRANES 


4.  Intermenstruum  (thirteen  days).  An  interval  of  rest.  ■ 

Since  ovulation  occurs  most  often  postmenstruum,  Grosser  believes  that  the  embryo 
reaches  the  uterus  during  the  premenstrual  stage.  The  congestion  and  loosening  of  the 
uterine  tissue  at  this  time  would  seemingly  favor  the  implantation  of  the  embryo,  and  the 
glandular  secretion  might  afford  nutriment  for  its  growth  until  implantation  occurred. 
The  first  phase  of  menstruation,  according  to  this  view,  prepares  the  uterine  mucosa  for 
the  reception  of  the  embryo.  If  pregnancy  supervenes,  it  soon  inhibits  any  further  premen- 
strual changes  so  that  menstruation  does  not  occur.  Menstruation  proper  would  then 
represent  an  over-ripe  condition  of  the  mucosa  and  the  abortion  of  an  unfertilized  ovum. 

The  Deciduae.  -The  intimate  fusions  between  fetal  and  maternal 
tissues  necessitate  an  extensive  sloughing  of  the  uterine  lining  at  birth. 


Fig.  50. — Vertical  section  through  the  decidua  vera  of  about  seven  months,  with  the  attached 
membranes  in  situ  (Schaper  in  Lewis  and  Stohr).  X 30. 

The  mucosa  of  the  pregnant  uterus  is,  therefore,  designated  the  decidua. 
Its  preparation  and  continuance  during  gestation,  and  the  long  deferred 
loss  and  repair  at  parturition,  only  exaggerate  the  events  of  an  ordinary 
menstrual  cycle.  The  two  processes  show  undoubted  fundamental 
similarities. 

The  chorionic  vesicle  lies  embedded  in  part  of  the  uterine  wall  only 
(Fig.  49).  This  allows  three  regions  to  be  recognized : (i)  a portion  not  in 
direct  contact  with  the  ovum,  the  decidua  vera;  (2)  a portion  which  con- 
stitutes a superficial  covering  or  arching  dome,  the  decidua  capsiilaris; 
(3)  a portion  underlying  the  embryo  and  between  it  and  the  muscularis, 
the  decidua  basalis. 


THE  DECIDUAL  MEMBRANES 


6l 

Decidua  Vera. — The  premenstrual,  superficial  compact  layer  and  deep 
spongy  layer  are  still  further  emphasized  in  pregnancy  (Fig.  50).  The 
compact  layer  contains  the  straight  but  dilated  segments  of  the  uterine 
glands.  Its  surface  epithelium  disappears  by  the  end  of  the  third  month. 
The  spongy  layer  is  characterized  by  the  greatly  enlarged  and  tortuous 
portions  of  the  glands  of  pregnancy. 

A prominent  constituent  are  the  decidual  cells  that  occur  chiefly  in 
the  stratum  compactum  (Fig.  50).  They  are  modified  stroma  cells, 


Fig.  51. — Diagrammatic  section  through  a pregnant  uterus  of  two  months  (Thomson). 
c,  Uterine  tube;  c',  mucous  plug  in  cervi.x;  dv,  decidua  vera;  dr,  decidua  capsularis;  ds,  decidua 
basalis;  ch,  chorion  (its  longer  villi  constitute  the  chorion  frondosum) ; am,  amnion;  ii,  umbilical 
cord;  al,  allantois;  y,y',  yolk  sac  and  stalk. 

frequently  multinucleate,  which  become  about  50^  in  diameter.  Athough 
diagnostic  of  pregnancy  their  function  is  in  doubt.  Many  degenerate 
during  the  later  months. 

During  the  first  two  months  of  gestation  the  long  axes  of  the  glands 
are  vertical.  Later,  as  the  decidua  is  stretched  and  compressed,  owing 
to  the  growth  of  the  fetus,  the  glands  are  broadened  and  shortened,  and 
their  cavities  become  elongated  clefts  parallel  to  each  other  and  to  the 
surface  of  the  decidua  (Fig.  50).  Similarly,  the  gland  cells  stretch,  and 
flatten  until  they  resemble  endothelium.  The  decidua  vera  attains  a 


62 


IMPLANTATION  AND  FETAL  MEMBRANES 


maximum  thickness  of  about  i cm.,  but  in  the  latter  half  of  pregnancy 
pressure  causes  it  to  thin  and  lose  much  of  its  early  vascularity.  The 
cervix  uteri  does  not  form  a decidua;  its  glands  secrete  a mucous  plug 
which  closes  the  uterus  until  the  beginning  of  labor  (Fig.  51). 

Decidua  Capsularis. — In  the  earlier  stages  of  development,  glands 
and  lilood  vessels  occur  in  its  substance  and  the  surface  epithelium  is 
continuous  with  that  of  the  decidua  vera  (Fig.  49).  As  the  chorion 
expands,  the  capsularis  grows  thin  and  atrophic.  During  the  fourth 
month  it  comes  into  contact  with  the  decidua  vera,  with  which  it  fuses, 
thereliy  obliterating  the  uterine  cavity  (Figs.  51  and  55).  Soon  after,  the 
capsularis  degenerates  and  disappears.  This  allows  the  chorion  Ireve  to 
become  adherent  to  the  decidua  vera  (Fig.  50). 

Decidua  Basal  is. — -IDuring  the  first  four  months  of  pregnancy  this 
portion  of  the  mucosa  resembles  the  decidua  vera  in  structure  (Fig.  49). 
Both  compact  and  spongy  layers  are  represented,  although  there  are  super- 
ficial erosions  and  blood  extravasations  caused  by  the  activity  of  the 
chorionic  trophoderm.  The  decidua  basalis  does  not  share  in  the  degen- 
eration common  to  the  other  deciduae  but  persists  until  birth  as  a compo- 
nent of  the  nutritional  organ  termed  the  placenta  (Figs.  52  and  55). 
The  decidua  is  said  to  help  in  preventing  excessive  hemorrhage  during 
the  earlier  part  of  pregnancy  by  acting  as  a dam  between  the  chorionic 
villi  and  the  eroded  uterus  (p.  58). 

The  Placenta 

The  ])lacenta  has  a double  origin.  The  chorion  frondosum  is  the 
fetal  portion  and  the  decidua  basalis  is  the  maternal  contribution  (Fig. 
49).  The  area  of  persistent  frondosum  villi  is  somewhat  circular  in  form, 
so  that  the  placenta  becomes  disc-shaped  (Fig.  56).  Near  the  middle  of 
its  fetal  surface  is  attached  the  umbilical  cord  ; the  surface  itself  is  covered 
by  glistening  amnion  that  has  fused  with  the  subjacent  chorion  (Fig.  52). 

The  Placenta  Fetalis. — The  villi  of  this  portion  of  the  chorion  form 
profusely  branched,  tree-like  structures  which  lie  in  the  intervillous  spaces 
(Figs.  52  and  54).  The  ends  of  some  of  the  villi  are  attached  to  the  wall 
of  the  decidua  basalis  and  are  known  as  anchoring  villi,  in  contrast  to  the 
floating  free  villi.  In  the  connective-tissue  core  of  each  villus  are  com- 
monly two  arteries  and  two  veins  (branches  of  the  umbilical  vessels), 
cells  like  lymphocytes,  and  special  ceils  of  Hofbauer  apparently  phago- 
cytic in  function.  Lymphatics  are  also  present.  The  epithelium  of  the 
villi  is  at  first  composed  of  a layer  of  trophectoderm,  with  the  outlines 
of  its  cuboidal  cells  sharply  defined  (Fig.  53  A).  This  layer  (of  Langhans) 
forms  and  is  covered  by  a syncytium,  the  trophoderm.  In  the  later 
months  of  pregnancy,  as  the  villi  grow,  the  trophectoderm  is  used  up  in 


THE  PLACENTA 


63 


forming  the  syncytium,  so  that  at  term  the  trophoderm  is  the  only  con- 
tinuous epithelial  layer  of  the  villi  (Fig.  53  B).  About  the  margin  of  the 
placenta  the  trophectoderm  persists  as  the  closing  ring,  which  is  con- 
tinuous with  the  epithelium  of  the  chorion  Ireve. 


The  Plaecnta  Materna. — This,  like  the  decidua  vera,  is  differentiated 
into  a basal  plate,  which  is  the  remains  of  the  compact  layer  and  forms  the 


64 


IMPLANTATION  AND  PETAL  MEMBRANES 


floor  of  the  intervillous  spaces,  and  into  a deep  spongy  layer  (Figs.  52  and 
54)- 

1 he  basal  plate  is  composed  of  a connective-tissue  stroma,  containing 
decidual  cells,  canalized  fibrin,  and  persisting  portions  of  the  epithelium 
of  the  villi.  The  ‘canalized  fibrin’  (Fig.  47)  forms  chiefly  by  a fibrinoid 


A 


B 


Fig.  53. — Transverse  sections  of  chorionic  villi  (Schaper  in  Lewis  and  Stohr).  A,  At  the  fourth 

week;  B,  C,  at  the  end  of  pregnancy. 

necrosis  of  the  mucosa,  but  the  fibrin  of  the  maternal  blood  and  the 
chorionic  trophoderm  also  participate  (Mall,  1915).  Septa  extend  from 
the  basal  plate  into  the  intervillous  spaces  but  do  not  unite  with  the 
chorion  frondosum  (Grosser).  Near  term,  these  constitute  the  septa 


THE  PLACENTA 


placent(E  (Fig.  54)  which  incompletely  divide  the  placenta  into  lobules,  or 
cotyledons  (Fig.  56  B). 

The  maternal  arteries  and  veins  pass  through  the  basal  plate,  taking 
a sinuous  course  and  opening  into  the  intervillous  spaces  (Fig.  54).  Near 
their  entrance  they  proceed  obliquety  and  lose  all  but  their  endothelial 
layers.  The  original  openings  of  the  vessels  into  the  intervillous  spaces 
were  formed  during  the  implantation  of  the  ovum  when  their  walls  were 
eroded  by  the  invading  trophoderm  of  the  villi  (Fig.  47).  As  the  placenta 
increases  in  size,  the  vessels  grow  larger.  The  ends  of  the  villi  frequently 
are  sucked  into  the  veins  and  interfere  with  the  placental  circulation. 

Uterine 

M nscularis  artery  Uterine  vein 


Intervillous 

space 


Syncytium 


Fig.  54. — Scheme  of  placental  circulation  (Kollmann).  Arrows  indicate  the  blood  flow  in 

the  intervillous  spaces. 

At  the  periphery  of  the  placenta  is  an  enlarged  intervillous  space  that 
varies  in  extent  but  never  circumscribes  the  placenta  completely.  This 
space  is  the  marginal  sinus  through  which  blood  is  carried  away  from  the 
placenta  by  the  maternal  veins  (Fig.  55).  The  blood  of  the  mother  and 
fetus  does  not  mix,  although  the  epithelial  cells  of  the  villi  are  instrumental 
in  transferring  nutritive  substances  to  the  blood  of  the  fetus  and  in  elimi- 
nating wastes  from  the  fetal  circulation  into  the  maternal  blood  stream  of 
the  intervillous  spaces. 


Uterine, 
artery  in 
septum 


Decidua 
basal  is 

Uterine  artery  in 
decidual  septum 


Umbilical  artery 


66 


IMPLANTATION  AND  FETAL  MEMBRANES 


Mall  (1915)  states  that  there  is  little  evidence  of  an  actual  intervillous  circulation; 
the  decidua  and  trophoderm  are  active  in  preventing  this  (pp.  58  and  62).  Some 
authorities  hold  that  the  intervillous  circulation  is  peculiar  to  the  second  half  of  pregnancy. 
In  summary,  Mall  regards  the  entire  question  as  still  open. 

Parturition 

Before  birth,  the  placenta  is  concave  on  its  amniotic  surface,  its  cur- 
vature corresponding  to  that  of  the  uterus  (Fig.  55).  At  term,  the  dura- 
tion of  which  is  taken  as  ten  lunar  months,  the  muscular  contractions  of 
the  uterus,  termed  ‘pains,’  bring  about  a dilation  of  the  cervix  uteri, 
the  rupture  of  the  amnion  and  chorion  lasve,  and  cause  the  extrusion  of  the 
child.  With  the  rupture  of  the  membranes  the  amniotic  liquor  is  expelled, 
but  the  fetal  membranes  remain  behind,  attached  to  the  deciduae.  The 


Fig.  56. — Mature  placenta  (Heisler).  A,  Entire  fetal  surface  with  membranes  attached  to  its 
periphery;  B,  detail  of  maternal  surface  showing  cotyledons. 


pains  of  labor  begin  the  detachment  of  the  decidual  membranes,  the  plane 
of  their  separation  lying  in  the  spongy  layer  of  the  decidua  basalis  and 
decidua  vera,  where  there  are  only  thin-walled  partitions  between  the 
enlarged  glands  (Figs.  50  and  52).  Following  the  birth  of  the  child,  the 
tension  of  the  umbilical  cord  and  the  ‘after  pains’  which  diminish  the 
size  of  the  uterus  normally  complete  the  separation  of  the  decidual  mem- 
branes from  the  wall  of  the  uterus.  The  uterine  contractions  serve  also 
to  diminish  the  size  of  the  ruptured  placental  vessels  and  prevent  exten- 
sive hemorrhage.  From  the  persisting  portions  of  the  spongy  layer  and 
from  the  epithelium  of  the  glands  are  regenerated'  the  tunica  propria, 
glands,  and  epithelium  of  the  uterine  mucosa. 


Fk;.  55. — Section  of  the  uterus,  illustrating  the  relation  of  an  advanced  fetus  to  the  placenta 

and  membranes  (Ahlfeld). 


t 


- 


PARTURITION 


67 


The  decidual  membranes,  and  the  structures  attached  to  them  when 
expelled,  constitute  the  ‘after  birth.’  The  placenta  is  disc-shaped,  about 
17  cm.  in  diameter,  2 cm.  thick,  and  weighs  500  gm.  It  is  usually  everted 
so  that  its  amniotic  surface  is  convex,  its  maternal  surface  concave  (Fig. 
56).  The  placenta  is  composed  of  the  amnion,  chorion  frondosum  (chori- 
onic villi  with  intervillous  spaces  divided  incompletely  by  the  septa  into 
cotyledons),  and  includes  on  the  maternal  side  the  basal  plate  and  a portion 
of  the  spongy  layer  of  the  decidua  basalis  (Fig.  52).  Near  the  center  is 
attached  the  umbilical  cord,  and  at  its  margins  the  placenta  is  continuous 
with  the  decidua  vera  and  the  remains  of  the  chorion  Iseve  and  decidua 
capsularis.  The  amnion  lines  all  the  deciduae  (Fig.  55). 

Gross  Changes  in  the  Uterus. — During  pregnancy  the  uterus  enlarges  enormously, 
due  chiefly  to  the  hypertrophy  of  its  muscle  fibers,  and  the  fundus  reaches  the  level 
of  the  xiphoid  process.  After  birth,  it  undergoes  rapid  involution;  at  the  end  of  one 
week  it  has  lost  one-half  its  weight,  and  in  the  eighth  week  the  return  is  complete. 
The  mucosa  is  regenerated  in  two  or  three  weeks  from  the  remains  of  the  spongy 
layer  (Fig.  52). 

Position  of  the  Placenta  and  Its  Variations. — The  position  of  the  placenta  is  determined 
by  the  point  at  which  the  embryo  is  implanted.  In  most  cases  it  is  situated  on  either 
the  dorsal  or  ventral  wall  of  the  uterus.  Occasionally  it  is  lateral  in  position,  and,  very 
rarely,  it  is  located  near  the  cervix  and  covers  the  internal  os  uteri,  constituting  a placenta 
prcBvia.  A partially  or  wholly  duplicated  placenta,  or  accessory  {succenturiate)  placentas 
may  be  formed  from  persistent  patches  of  villi  on  the  chorion  Iseve. 

Ectopic  Pregnancy. — If  the  ovum  becomes  implanted  and  develops  elsewhere  than 
in  the  uterus,  the  condition  is  known  as  an  extra-uterine,  or  ectopic  pregnancy.  The 
commonest  site  is  the  uterine  tube,  tubal  pregnancy.  Attachment  to  the  peritoneum, 
abdominal  pregnancy,  and  the  development  of  an  unexpelled  ovum  within  the  ruptured 
follicle,  ovarian  pregnancy,  are  known  also. 

Plural  Pregnancy. — Twins  occur  once  in  85  births;  triplets,  once  in  7000;  quadruplets, 
once  in  750,000.  Each  member  of  ordinary  double-ovum  ‘twins’  (p.  42)  has  its  own 
amnion,  chorion,  and  umbilical  cord.  The  placenta  and  decidua  capsularis  are  also  indi- 
vidual, except  in  those  cases  where  the  original  proximity  of  implantation  leads  to  sec- 
ondary fusions.  Single-ovum,  identical  twins  comprise  only  15  per  cent  of  the  entire 
twin  group;  the  chorion,  placenta,  and  decidua  capsularis  are  necessarily  common,  but 
the  cord  and  usually  the  amnion  are  double. 


CHAPTER  IV 


AGE,  BODY  FORM  AND  GROWTH  CHANGES 
AGE.  SIZE  AND  WEIGHT  OF  EMBRYOS 

The  age  of  a human  embryo  can  not  be  determined  with  certainty, 
because  too  little  is  known  of  the  time  relations  existing  between  ovulation 
and  menstruation,  and  between  ovulation,  coitus,  and  fertilization  (p. 
27).  This  lack  of  a reliable  basis  makes  any  computation  approximate, 
although  the  errors  thus  introduced  are  significant  only  in  young  specimens. 

From  numerous  clinical  observations  it  is  certain  that  ovulation  does 
not  immediately  precede  menstruation,  as  was  long  held,  but  on  the 
contrary  follows  it  (p.  24).  Experience  proves  that  most  pregnancies 
date  from  a coitus  within  a week  or  ten  days  after  the  menses  cease.  Hence, 
it  is  a])proximately  correct  to  compute  the  age  of  an  embryo  from  the 
tenth  Jay  after  the  onset  of  the  last  menstruation. 

Careful  studies  on  embryos  which  were  accompanied  by  adequate 
data  as  to  menstruation,  coitus,  and  clinical  history  have  led  to  the  estab- 
lishment of  certain  age-norms.  By  comparing  a given  specimen  with 
such  standards  its  age  can  be  determined  with  reasonable  accuracy. 
It  is  simplest  to  make  these  comparisons  on  the  basis  of  size,  although 
young  embryos  vary  sufficiently  so  that  structure  must  be  taken  into 
account  as  well.  Embryos  are  measured  in  two  ways.  Commonest  is 
the  crown-rump  length  (designated  CR),  or  sitting  height;  this  is  the  mea- 
sure from  vertex  to  breech.  The  second  is  the  crown-heel  length  (CH).  or 
standing  height. 

The  following  table,  based  on  data  by  Mall  and  Scammon,  lists  the 
size  and  weight  of  human  embryos  corresponding  to  definite  ages : 

Ratio  of  increase 


Crown-rump  length 

Crown-heel  length 

to  weight  at 

(.CR),  or  sitting 

(CH).  or  standing 

Weight  in 

beginning  of 

Age 

height  (mm.). 

height  (mm.) 

grams 

month 

Three  weeks 

0.5 

0.5 

Four  weeks 

2.5 

2-5 

8000.00 

Five  weeks 

5-5 

5-5 

. 004 

Six  weeks 

I I . 0 

11.0 

.Seven  weeks 

17.0 

19 . 0 

Second  lunar  month 

25.0 

30. 0 

2 

499 . 00 

Third  lunar  month 

68.0 

98 . 0 

24 

I I . 00 

Fourth  lunar  month 

I2I  . 0 

180.0 

120 

4.00 

Fifth  lunar  month 

167 . 0 

250.0 

330 

1-75 

Sixth  lunar  month 

210.0 

3130 

600 

0.82 

Seventh  lunar  month ... 

245 . 0 

370.0 

1000 

0.67 

Eighth  lunar  month 

284.0 

425  0 

1600 

0.60 

Ninth  lunar  month 

316.0 

470.0 

2400 

0.50 

Tenth  lunar  month 

345  ■ 0 

500. 0 

3200 

0.33 

68 


AN  OUTLINE  OF  PRENATAL  DEVELOPMENT  69 

For  estimating  the  age  of  an  embryo  when  its  size  is  known,  or  the 
reverse,  the  following  rules  are  useful ; 

Standing  height  (in  cm.)  X 0.2  = Age  {in  months) 

Sitting  height  (in  cm.)  X 0.3  = ,-lg^  {in  months) 

(For  embryos  less  than  10  cm.  long,  add  one  month  to  the  result) 

Age  (in  months)  ^ 0.2  = Standing  height  (in  cm.) 

Age  {in  months)  -u  0.3  = Sitting  height  {in  cm.) 

(For  embryos  of  the  first  3 months,  subtract  4 cm.  from  the  result) 

Of  practical  interest  is  the  determination  of  the  date  of  delivery 
of  a pregnant  woman.  Most  labors  occur  ten  lunar  months,  or  280  days, 
from  the  first  day  of  the  last  menstrual  period.  The  month  and  day  of 
this  date  are  easily  found  by  counting  back  three  months  from  the  first 
day  of  the  last  period,  and  then  adding  one  week.  As  some  women  men- 
struate once  or  more  after  becoming  pregnant  this  computation  is  not 
infallible. 

For  comparison  and  reference,  the  gestation  periods  of  a few  repre- 
sentative mammals  are  appended  ; 

Opossum 13  days  Pig 

Mouse 20  days  Sheep 

Rat 21  days  Cow 

Rabbit 30  days  Horse 

Cat 8 W’eeks  Rhinoceros 

Dog,  guinea  pig 9 weeks  Elephant . . 

AN  OUTLINE  OF  PRENATAL  DEVELOPMENT 

The  early  history  of  the  human  ovum,  including  implantation  and  the 
development  of  membranes  for  its  protection  and  nutrition,  has  been 
described  on  previous  pages.  The  present  section  will  deal  with  the 
appearance  of  the  embryo  and  fetus  at  successive  stages  of  uterine  existence. 

Period  of  the  Embryo 

Embryos  of  the  Second  Week. — The  youngest  known  embryo  is  the 
Miller  specimen.  It  is  somewhat  like  the  diagram  represented  in  Fig. 
40  A.  The  central  embryonic  anlage  is  solid,  without  amnion  cavity  or 
yolk  sac;  it  measures  o.i  mm.  in  length.  The  extra-embryonic  mesoderm 
is  unsplit  by  a coelom.  The  chorion  has  both  syncytial  and  Langhans 
layers,  but  true  mesodermal  villi  are  absent;  its  internal  cavity'  measures 
0.44  mm. 

The  Bryce-Teacher  ovum  (Fig.  40  B)  differs  from  the  foregoing  speci- 
men chiefly  by  possessing  an  amniotic  cavity^  and  y'olk  sac. 

A well-defined  extra-embryonic  coelom  divides  the  mesoderm  of 
Peter’s  specimen  into  somatic  and  splanchnic  lay^ers,  and  there  is  also  the 


17  weeks 
21  weeks 
41  weeks 
48  weeks 

18  months 
20  months 


70 


AGE,  BODY  FORM  AND  GROWTH  CHANGES 


beginning  of  true  villi  (Figs.  40  C and  46).  The  ectodermal  embryonic 
disc  measures  0.19  mm.;  it  is  thickened  and  separated  from  the  entoderm 
by  a layer  of  mesoderm  (Fig.  41).  Strands  of  mesoderm,  known  as  the 
magma  reticulare,  bridge  the  extra-embryonic  body  cavity,  which  is  0.9 
X 1.6  mm.  in  diameter  (Fig.  41). 

These  ova  all  belong  to  the  latter  part  of  the  second  week.  The  yolk 
sac  is  smaller  than  the  amnion  and  the  villi  are  mostly  unbranched.  The 
embryo  is  merely  a plate  combined  from  the  three  germ  layers.  Neither 
primitive  streak  nor  allantois  has  appeared.  Even  in  the  oldest,  a broad 
zone  of  mesoderm  connects  embryo  to  chorion. 

Embryos  of  the  Third  Week. — The  Mateer  ovum  is  shown  as  Fig. 
32  A.  It  possesses  a distinct  primitive  groove  and  allantois.  The 
embryonic  disc  is  0.9  mm.  in  length. 


To!/;  sac 


A mn  'iOn 


Neural  groove 


Nciircnleric  canal 
Primitive  streak 
Body 


FiO-  57 — Dorsal  view  of  a human  embryo  of  1.54  mm.  (Spee).  X 23. 

A head  process  with  its  contained  notochordal  canal  features  the 
advance  illustrated  by  the  Ingalls  embryo  (Fig.  32  B).  There  is  also  the 
beginning  of  a neural  groove.  The  chorionic  vesicle  has  an  internal 
diameter  of  7 mm. 

Spec’s  specimen  has  progressed  still  further  (Fig.  57).  The  embry- 
onic disc  measures  1.54  mm.  and  is  slightly  constricted  from  the  yolk  sac. 
The  primitive  streak  is  confined  to  the  caudal  end  of  the  embryonic  disc, 
the  neural  folds  are  well-marked,  and  a neurenteric  canal  opens  as  a pore 
into  the  primitive  intestinal  cavity.  In  longitudinal  section  it  is  evident 
that  the  floor  of  the  head  process  has  disappeared,  leaving  its  roof  as  the 


AN  OUTLINE  OF  PRENATAL  DEVELOPMENT 


71 


notochordal  plate  (Figs.  40  D and  43).  The  fore-gut  is  forming  and  there 
are  indications  of  a future  heart  anlage. 

In  this  group  as  a whole,  the  continued  extension  of  the  extra-embry- 
onic coelom  has  separated  the  embryo  from  the  chorion  except  in  the  region 
of  the  body  stalk,  which  constitutes  a bridge  that  contains  the  allantois. 
The  yolk  sac  is  now  larger  than  the  amnion.  The  chorionic  villi  branch 
freely  and  there  is  evidence  of  blood-vessel  formation  in  the  wall  of  the 
yolk  sac  (Fig.  43),  and,  usually,  in  the  body  stalk  and  chorion. 

Embryos  of  the  Fourth  Week. — Embryos  of  this  period  are  early 
characterized  by  the  presence  of  high  neural  folds  (Fig.  58)  whose  edges 
soon  unite  along  part  of  their  extent  to  form  a tube  which  is  the  anlage  of 
the  brain  and  spinal  cord  (Figs.  59  and  245).  The  expansive  brain  portion 
is  already  recognizable.  The  mesoderm 
of  each  side  of  the  midplane  becomes 
arranged  in  blocks,  the  primitive  (mes- 
odermal) segments,  visible  externally. 

In  the  embryo  shown  in  Fig.  245  there 
are  14  pairs.  The  primitive  streak  is 
now  insignificant  (Figs.  44  and  s8).  , 

' ' Cut  edge  of 

amnion 

Primitive 
segments 


Neural  folds 


Neurenleric  canal 


Fig.  58. — Kromer  human  embryo  of  1.8 
mm.,  in  dorsal  view  (after  Keibel  and  Elze). 
X 20. 


Fig.  59. — Human  cmbrjm  of  2.1 1 mm.  in 
dorsal  view  (Eternod).  X 35. 


Growth  at  the  head  and  tail  regions  appears  to  constrict  the  embryo  from 
the  yolk  (Figs.  58  and  245).  In  a longitudinal  section  of  an  embryo  at  the 
middle  of  this  period  (Fig.  44),  both  fore-  and  hind-gut  are  evident  and 
the  heart  is  conspicuous.  A system  of  blood  vessels  is  established  connect- 
ing with  the  heart  (Figs.  180  and  181).  The  embryo  is  now  cylindrical, 
its  body  wall  encloses  two  more  or  less  complete  tubes  (neural  and  enteric) 
with  the  axial  notochord  between.  During  this  period  there  is  an  increase 
in  length  from  0.5  to  2.5  mm. 

Embryos  of  the  Fifth  Week. — Specimens  corresponding  to  Figs. 
60  and  61  stand  at  the  turn  between  the  fourth  and  fifth  weeks,  whereas 


72 


AGE,  BODY  FORM  AND  GROWTH  CHANGES 


one  like  Fig.  62  is  more  representative  of  this  period.  The  progressive 
separation  of  embryo  from  yolk  sac  is  evident.  The  primitive  segments 
have  increased  until  the  2.6  mm.  specimen  (Fig.  61)  has  35  of  the  defini- 


tive 38  pairs.  The  convex  curvature  of  the  back  is  characteristic.  Exter- 
nal swellings  indicate  the  three  primary  brain  vesicles  and  the  head 
becomes  hexed  at  a right  angle  in  the  mid-brain  region.  On  each  side 


of  the  future  neck  appear  branchial  arches,  separated  by  grooves.  The 
hrst  pair  of  arches  bifurcates  into  maxillary  and  mandibular  processes 
that  will  form  the  upper  and  lower  jaws;  between  them  is  a depression, 


AN  OUTLINE  OF  PRENATAL  DEVELOPMENT 


73 


the  oral  fossa  or  stomodeum,  where  the  mouth  will  be.  The  heart  is  large 
and  flexed.  The  body  ends  in  a blunt  tail,  and,  toward  the  end  of  the 
period,  bud-like  outgrowths  indicate  the  anlages  of  the  upper  and  lower 
limbs.  An  idea  of  the  extent  of  internal  organization  may  be  gained  by 
examining  Figs.  91,  183  and  184. 


Embryos  of  Six  to  Eight  Weeks. — These  embryos  range  between 
5.5  and  25  mm.  and  show  marked  changes.  Their  external  form  comes  to 
resemble  more  the  adult  condition,  and,  after  the  second  month,  the  devel- 
oping young  is  designated  a fetus.  This  external  metamorphosis  may  be 
followed  by  referring  to  the  illustrations  of  embryos  of  7 mm.  (Fig.  63), 
9 mm.  (Fig.  227),  12  mm.  (Fig.  64),  18  mm.  (Fig.  65),  and  23  mm.  (Fig. 
66;  two  months).  It  is  due  principally  to  the  following  factors:  (i) 
Changes  in  the  flexures  of  the  body;  the  dorsal  convexity  is  lost,  the 
head  becomes  erect,  and  the  body  straight.  (2)  The  face  develops  (also 
illustrated  in  Fig.  68).  (3)  The  external  structures  of  the  eye,  ear,  and 

nose  appear.  (4)  The  prominent  tail  of  the  sixth  week  regresses  and 
becomes  inconspicuous,  largely  through  concealment  by  the  growing 
buttocks.  (5)  The  umbilical  cord  encloses  both  yolk  stalk  and  body 
stalk  and  constitutes  the  sole  attachment,  limited  to  the  region  of  the 
umbilicus.  (6)  The  heart,  which  formed  the  chief  ventral  prominence  in 
earlier  embryos,  now  shares  this  distinction  with  the  rapidly  growing  liver, 
and  the  two  determine  the  ventral  body  shape  until  the  eighth  week 
when  the  gut  dominates  the  belly  cavity  and  the  contour  of  the  abdomen 
is  more  evenly  rotund.  (7)  The  appearance  of  a neck  region,  due  chiefly 


Mid-brain 


nody  stalk 

Fig.  62. — Human  embryo  of  4.2  mm.  (His).  X 15. 


74 


AGE,  BODY  FORM  AND  GROWTH  CHANGES 


Myelencephalon 


Spinal  cor 


Cervical  segment  S 


Future  milk  line 


M etence  phalon 


Mesencephalon 


Diencephalon 


Yolk  sac  and 
umbilical  cord 


Thoracic  segment 


Lumbar  segment  5 

Fig.  63. — Human  embryo  of  7 mm.  (Mall  in  Kollman).  X 14.  1,  II,  HI,  Branchial  arches; 

II,  lit.,  heart;  L,  liver;  0,  otic  vesicle;  R,  olfactory  placode. 


Fig.  64. — Human  embryo  of  12  mm.  (Prentiss).  X 4- 


External  ear 
Mandibular  process 
Upper  limb  bud 
Mesodermal 

Lower  limb  bud 


Maxillary  process 
Attachment  of  amnion 


Yolk  stalk 


Yolk  sac 


AN  OUTLINE  OF  PRENATAL  DEVELOPMENT 


75 


to  the  settling  of  the  heart  caudad  and  the  loss  of  the  branchial  arches. 
(8)  The  external  genitalia  appear  in  their  ‘sexless’  condition. 

Period  of  the  Fetus 

During  the  third  month  the  fetus  definitely  resembles  a human  being, 
but  the  head  is  still  disproportionately  large  (Fig.  66);  the  umbilical 
herniation  is  reduced  by  the  return  of  the  intestine  into  the  abdomen; 
the  eyelids  fuse,  nail  anlages  form,  and  sex  can  now  be  distinguished 
readily.  In  the  fourth  month,  the  muscles  become  active  and  cause  fetal 
movements;  lanugo  hair  makes  its  appearance  (Fig.  66).  At  five 
months,  hair  is  present  on  the  head.  During  the  sixth  month  the  eye 
brows  and  lashes  grow  and  vernix  caseosa  forms;  the  body  is  lean  but 


Fig. 


65. — Human  embryo  of  18  mm.  with  its  membranes.  X 2.  The  chorion  is  opened  and 
reflected;  the  upper  half  of  the  amnion  has  been  cut  away. 


in  better  proportion.  At  seven  months,  the  fetus  looks  like  a dried-up, 
old  person  with  red,  wrinkled  skin;  the  eyelids  reopen.  In  the  eighth 
month,  the  testes  usually  are  in  the  scrotum ; infants  of  this  age  born 
prematurely  may  generally  be  reared.  In  the  ninth  month,  the  dull 
redness  of  the  skin  fades,  wrinkles  smooth  out,  the  panniculus  adiposus 
develops,  the  limbs  become  rounded,  and  nails  extend  to  the  finger  tips. 
At  ten  months,  the  child  is  ‘at  full  term,’  ready  to  cope  with  an  extra- 
uterine  existence  (Fig.  55). 


?6 


AGE,  BODY  F(JRM  AND  GROWTH  CHANGES 


GO  days 


Fig.  66. — Human  embryos  of  three  weeks  to  two  months  (His),  and  fetuses  of  three  and 

four  months  (De  Lee).  Natural  size. 


THE  ESTABLISHMENT  OF  EXTERNAL  FORM 
Although  the  preceding  section  deals  largely  with  the  aquisition  of 
fetal  form,  this  topic  requires  supplementary  treatment. 

The  Head  and  Neck 

Since  development  in  the  cephalic  region  maintains  its  early  advan- 
tage, the  head  and  neck  of  an  embryo  are  for  a long  time  disproportionate^ 
large.  In  Fig.  63  the  last  cervical  segment  is  midway  on  the  body.  The 
gradual  adjustment  of  size  relations  may  be  traced  in  Fig.  6g 


Anomalies.—  Many  grossly  abnormal  embryos  are  found  at  operation  or  spontaneous 
abortion.  Various  pathological  conditions  in  the  embryo  commonly  accompany  those 
disturVjances  which  induce  its  stunting  or  death.  Degenerative  changes  are  common  also 
in  the  fetal  membranes,  although  the  chorionic  sac  sometimes  continues  to  grow  quite 
normally  after  the  embryo  has  died  or  disappeared.  Dead,  retained  fetuses  are  usually 
resorbed,  but  they  may  mummify  and  ])ersist  indefinitely. 


25  days 


THE  ESTABLISHMENT  OE  EXTERNAL  FORM 


77 


The  head  is  composed  of  two  portions  almost  from  the  start.  One  is 
neural  in  nature  and  includes  the  brain,  eyes,  and  internal  ears,  and  their 
supporting  structures.  The  other  is  the  facial,  or  visceral,  part  that 
contains  the  cephalic  ends  of  the  alimentary  and  respiratory  tracts.  The 
neural  portion  is  much  the  larger  in  young  embryos  and  this  superiority 
is  never  lost  completely,  although  the  subsequent  differentiation  and 
growth  of  the  nose,  jaws,  and  pharynx  reduces  the  early  disparity. 

Branchial  Arches. — -The  formation  of  the  face  and  neck  involves  the 
history  of  the  branchial  arches.  These  are  bar-like  prominences,  sepa- 
rated by  grooves,  which  occur  on  the  lateral  surfaces  of  the  neck  (Figs. 
6 1 to  63).  They  correspond  to  the  gill-bearing  arches  of  fishes  that  are 
separated  by  clefts  through  which  respiratory  water  flows.  In  amniotes 
they  never  assume  a respiratory  function,  but  occur  as  transitory  vestiges 
that  are  applied  to  various  purposes,  then  disappear.  The  human  embryo 
develops  five  such  arches,  separated  by  four  ectodermal  grooves;  sub- 
jacent to  these  grooves  the  entoderm  of  the  pharynx  bulges  correspond- 
ingly (Fig.  87).  The  thin  plates  thus  formed  by  the  union  of  ecto- 
derm and  entoderm  sometimes  rupture  to  make  temporary  openings, 
reminiscent  of  the  gill-slit  condition. 

The  last  arch  lies  caudal  to  the  fourth  cleft  and  is  poorly  defined 
along  its  posterior  margin.  Toward  the  end  of  the  sixth  week,  the  first 
and  second  arches  overlap  the  other  three  and  ob.scure  them.  Fig.  63 
shows  the  beginning  of  this  process.  Fig.  227  an  advanced  stage,  and  in 
Fig.  64  it  is  complete.  The  caudal  arches  sink  into  a triangular  depression 
called  the  cervical  sinus.  When  the  posterior  edge  of  the  second  arch 
fuses  with  the  thoracic  wall,  the  sinus  and  its  contained  arches  are  closed 
off.  This  cavity  eventually  degenerates. 

Various  muscles  and  bones  form  from  the  arches,  and  from  the  ento- 
dermal  pouches  certain  glandular  organs  arise.  The  completion  of  this 
metamorphosis  marks  the  appearance  of  a neck  (Fig.  65)  which  is  charac- 
teristic of  amniotes  alone. 

Anomalies. — Imperfect  closure  of  the  branchial  clefts  (usually  the  second)  leads  to 
the  formation  of  cysts,  diverticula,  or  even  fistulfe.  Such  structures  may  be  derived  either 
from  an  ectodermal  groove  or  the  complementary  entodermal  pouch. 

The  Face. — Pig  embryos  show  clearly  how  the  face  forms.  In  Fig. 
369  the  expansive  jronto-nasal  process  represents  much  of  the  front  of 
the  head.  The  olfactory  pits  are  present,  and  the  first  branchial  arches 
have  not  only  bifurcated  into  maxillary  and  mandibular  processes  but  the 
mandibular  segments  have  already  united  as  the  lower  jaw.  Laterally, 
the  olfactory  pits  subdivide  the  fronto-nasal  process  into  paired  lateral 


78 


AGE,  BODY  FORM  AND  GROWTH  CHANGES 


and  median  nasal  processes  (Figs.  394  and  67  A).  Soon,  the  median  nasal 
processes  fuse  with  each  other  and  with  the  maxillary  processes;  this 
constitutes  the  upper  jaw  (Fig.  67  B).  The  lateral  nasal  processes  like- 
wise join  the  maxillary  process,  thereby  obliterating  the  lacrimal  groove, 
and  forming  the  wings  and  margins  of  the  nose  and  the  adjacent  cheek 
region.  Meanwhile,  the  mesial  portion  of  the  original  fronto-nasal 
process  becomes  the  forehead  and  the  septum  and  bridge  of  the  nose. 

The  early  development  of  the  human  face  is  essentially  the  same. 
These  changes  may  be  followed  in  Fig.  68.  At  first  the  nose  is  broad  and 
fiat,  with  the  nostrils  set  far  apart  and  directed  forward  (Fig.  68  C).  In 


Eye 

Lacrimal  groove 
Maxillary  process 

Branchial  groove  i 
Branchial  groove,  2 
A 


Lateral  nasal  process 
Olfactory  pit 

Median  nasal  process  — ^ 

Ma?tdible 

Branchial  arch  2 
Ventral  aorta 


Fig.  67. — Development  of  the  pig’s  face  (Prentiss).  X 7.  A,  12  mm.;  B,  14  mm. 

the  later  fetal  months  the  bridge  is  elevated  and  prolonged  into  the 
apex,  and  the  nostrils  look  downward  (Fig.  68  D).  The  line  of  fusion  of 
the  median  nasal  processes  is  evident  in  the  adult  as  the  philtrum . The 
chin  is  a median  projection  from  the  fused  mandibular  processes.  During 
the  formation  of  the  jaws  the  originally  broad  mouth  opening  is  reduced 
in  its  lateral  extent.  Epithelial  ingrowths  begin  to  separate  the  lips 
from  the  alveolar  portions  of  the  jaws  at  the  fifth  week  (Fig.  79);  at 
birth  the  inner  edges  of  the  lips  bear  numerous  villosities.  Progressive 
modelling  of  the  face  continues  until  the  individual  becomes  fully 
grown. 


THE  ESTABLISHMENT  OF  EXTERNAL  FORM 


79 


Anomalies. — A common  facial  defect  is  hare  lip.  This  is  usually  unilateral  and  on  the 
left  side.  It  may  involve  both  lip  and  maxilla.  Hare  lip  is  attributed  to  the  failure  to 
fuse  of  the  median  nasal  and  maxillary  processes  (Kdlliker),  or  the  lateral  and  median  nasal 
processes  (Albrecht). 


Fig.  68. — Stages  in  the  development  of  the  human  face  (adapted).  A,  Five  weeks;  B, 
six  weeks;  C,  eight  weeks;  D,  si.xteen  weeks.  The  fronto-nasal  process  is  indicated  by  parallel 
lines,  the  median  nasal  processes  by  circles,  and  the  lateral  nasal  processes  by  dots. 

The  Sense  Organs. — The  eye,  ear,  and  nose  will  be  considered  in 
detail  in  Chapter  XV.  The  external  nose  has  just  been  described.  The  eye 
makes  its  appearance  in  the  early  weeks,  and,  by  the  second  month,  lids 
are  present.  For  a time  the  e^-es  are  placed  laterally  and  far  apart,  but 


8o 


AGE,  BODY  FORM  AND  GROWTH  CHANGES 


gradually  this  distance  is  reduced  (Fig.  68).  The  external  ear  is  developed 
around  the  first  branchial  groove  by  the  appearance  of  small  tubercles  which 
form  the  auricle  (Figs.  64,  65  and  31 1).  The  groove  itself  becomes  the 
external  auditory  meatus. 

The  Trunk 

In  young  embryos  the  trunk  is  like  a cylinder,  flattened  by  lateral 
compression  (Fig.  63).  Its  external  contour  is  determined  by  the  model- 
ling of  the  viscera  within.  During  the  fetal  period,  this  visceral  mass 
becomes  more  rounded  and  the  muscles  and  skeleton  of  the  trunk  appear, 
d'he  trunk  then  assumes  an  ovoid  form,  circular  in  section,  and  largest  at 
the  umbilicus  (Fig.  66).  From  the  third  fetal  month  through  early 
infancy  there  is  relative!}^  little  change  in  the  trunk  ])roportions.  When 
erect  i)osture  is  assumed,  the  dominance  of  the  thorax  and  abdomen  is 
reduced  and  the  lumbar  region  gains  in  prominence  and  relative  length. 
The  thorax  of  the  newborn  is  rather  conical,  with  its  base  below,  due  to 
the  ril:)s  l)cing  more  horizontal.  In  the  adult  the  thorax  is  barrel-shaped, 
that  is,  broadest  in  its  middle.  The  characteristic  curves  of  the  spinal 
column  are  absent  at  birth.  They  appear  partly  through  the  drag  of  body 
weight,  ])artly  through  the  pull  of  the  muscles,  and  are  not  pronounced 
until  the  posture  becomes  erect. 

Anomalies.—  The  embryonic  tail  sometimes  persists  and  develops  beyond  its  ordinary 
size.  Specimens  as  long  as  8 cm.  have  been  recorded  in  the  newborn.  Most  are  soft  and 
ileshy,  but  a few  have  contained  skeletal  elements.  Some  tumors  of  the  coccygeal  region 
are  attributed  to  the  activity  of  residual  primitive-streak  tissue. 

The  Appendages 

The  limbs  appear  during  the  fifth  w^eek  as  lateral  buds.  In  a 4 mm. 
embryo  (Fig.  62)  limb  buds  may  be  recognized,  but  due  to  the  early 
expanse  of  the  head-neck  region  they  seem  to  be  located  far  down  the  body. 
The  distal  ends  flatten  (Fig.  63)  and  a constriction  divides  this  paddle- 
like portion  from  the  proximal,  rounded  segment  (Fig.  196).  Later,  a 
second  constriction  separates  the  cylindrical  part  into  two  further  segments 
(Figs.  64  and  65),  and  the  three  divisions  of  arm,  forearm,  and  hand,  or 
thigh,  leg,  and  foot  are  respectively  formed.  Radial  ridges,  separated  by 
grooves,  first  foretell  the  formation  of  digits  (Figs.  64  and  65).  These 
elongate  as  the  definitive  fingers  and  toes,  and  rapidly  project  beyond  the 
original  plates;  the  latter  by  a slower  rate  of  growth  become  confined  as 
webs  about  the  basal  ends  of  the  digits  (Fig.  66).  The  thumb  and  great 
toe  early  separate  widely  from  the  index  finger  and  second  toe. 

The  limbs  as  a wdiole  undergo  several  changes  of  position.  At  the 
very  start  they  point  caudad  (Figs.  ig6  and  64),  but  soon  project  outward 
at  right  angles  to  the  bod\^  wall.  Next,  they  are  bent  ventrad  so  that  the 


GROWTH  CHANGES 


thumb  (radial)  side  of  the  arm  and  the  great  toe  ( tibia!)  side  of  the  leg  are 
directed  forward;  the  palmar  and  plantar  surfaces  face  the  body;  the 
elbow  turns  outward  and  somewhat  caudad,  the  knee  outward  and 
slightly  cephalad  (Fig.  65).  Finally,  both  sets  of  limbs  undergo  a torsion 
of  90°  about  their  long  axes,  but  in  opposite  directions.  As  a result,  the 
radial  side  of  the  arm  is  outward  (when  radius  and  ulna  are  parallel)  and 
the  palm  faces  ventrad;  on  the  contrary,  the  tibal  side  of  the  leg  is  the 
inner  side,  while  the  sole  faces  dorsad.  By  following  through  these  changes 
it  will  be  seen  that  the  radial  and  tibial  sides  of  arm  and  leg  are  homologous, 
as  are  palm  and  sole,  elbow  and  knee. 

The  upper  limb  buds  arise  first  and  they  maintain  a slight  advance  in 
differentiation.  Not  until  the  second  year  of  childhood  are  the  two  equal 
in  length. 

Anomalies. — The  extremities  may  either  fail  to  develop,  or  become  mere  stubs;  the 
hands  and  feet  may  join  the  body  like  flippers.  Rarely,  the  hands  or  feet  are  partially 
duplicated  or  reduced.  The  presence  of  extra  digits  is  polydactyly;  a fusion  of  digits  con- 
stitutes syndactyly.  More  or  less  complete  union  of  the  legs  occurs  as  sympodia. 

GROWTH  CHANGES 

The  developmental  period  of  man  is  divided  by  the  incident  of  birth 
into  prenatal  and  postnatal  periods.  At  birth  the  infant  is  sufficiently 
advanced  to  be  cared  for  outside  its  mother’s  body,  yet  its  development 
is  far  from  complete.  In  its  new  environment  differentiation  and  growth, 
especially  marked  by  changes  in  form  and  proportion,  continue  until  the 
beginning  of  the  third  decade ; only  then  is  full  size  and  mature  structure 
attained. 

The  several  divisions  of  the  developmental  period  are  listed  as  follows 
by  Scammon,  from  whose  account  much  of  the  material  of  the  succeeding 
paragraphs  is  taken : 


Divisions  of  the  Develop.mental  Period  in  Man 

^Period  of  the  ovum  (Fertilization  to  end  of  second  week) 

Prenatal  life  - Period  of  the  embryo  (Second  to  eighth  week) 

\Period  of  the  fetus  (Second  to  tenth  month) 

Birth 

Period  of  the  newborn  (Neonatal  period;  birth  to  end  of  second  week) 

1 Infancy  (Second  week  until  assumption  of  erect  posture  at  13  to  14  months) 
j 1 Early  childhood  (Milk-tooth  period;  first  to  sixth  year) 

j ’idliood  childhood  (Sixth  to  ninth  or  tenth  year) 

” \ Later  childhood  (Prepubertal  period;  from  9 or  10  years  to  12-15 

Postnatal  life  ! I years  in  females  and  13-16  years  in  males) 

I Puberty 

I (Fourteenth  year  in  females:  sixteenth  year  in  males) 

I Adolescence  (From  puberty  to  the  last  years  of  the  second  decade  in  females 
I and  to  the  first  years  of  the  third  decade  in  males) 


6 


82 


AGE,  BODY  FORM  AND  GROWTH  CHANGES 


Changes  in  Form.  ^If  an  adult  maintained  the  chubby  newborn  shape 
his  weight  would  be  twice  the  actual  amount.  Fig.  69  shows  the  propor- 
tions of  the  body  at  various  developmental  periods,  all  drawn  as  of  the 
same  height.  Note:  the  great  decrease  in  the  size  of  the  head;  the  con- 
stancy of  the  trunk  length;  the  early  completion  of  the  arms  and  the 
tardier  growth  of  the  legs;  the  upward  shift  of  the  umbilicus  and  symphy- 
sis pubis,  and  the  downward  trend  of  the  midpoint  of  the  body. 


2 mo.  (fa-tal)  s mo.  Newborn  2 yrs.  6 yrs.  12  yrs.  25  yrs. 


Fig.  6g. — Diagram.s  to  illustrate  the  changing  proportions  of  the  body  during  prenatal  and  post- 
natal growth  (Scammon  after  Stratz). 

Certain  of  these  facts  may  be  tabulated  in  terms  of  per  cent  of  the 
total  body  volume: 

Growth  in  Relative  Volume  of  the  Parts  of  the  Body 


In  per  cent  of  the  total  body  volume 


Age 

Head  and  neck 

Trunk 

Arms 

Legs 

Second  fetal  month 

45 

50 

3 

3 

Sixth  fetal  month 

37 

40 

8 

15 

Birth 

27 

49 

9 

15 

Two  years 

22  1 

50.5 

9 

17-5 

Six  years 

15 

51 

9 

25 

^Maturity 

7 

53 

10 

30 

Increase  in  Surface  Area. — The  relation  of  surface  area  to  body 
mass  or  volume  has  a profound  influence  on  metabolism.  This  relation 
changes  greatly  during  the  postnatal  period.  At  birth,  the  surface  area  is 
about  2500  sq.  cm.  This  is  doubled  in  the  first  year,  tripled  by  the  mid- 
dle of  childhood,  and  increases  rapidly  before  puberty.  At  maturity,  the 
total  gain  is  seven-fold.  Since,  however,  the  weight  of  the  body  has 
increased  some  twenty-fold  in  the  same  time  it  is  obvious  that  there  has 


GROWTH  CHANGES 


83 


been  a relative  loss.  Thus,  in  the  newborn  there  are  over  800  sq.  cm.  per 
kilogram  of  body  weight,  whereas  in  the  adult  there  are  less  than  300 
sq.  cm. 

Growth  in  Weight. — During  prenatal  life  the  weight  of  the  body 
increases  several  billion  times,  whereas  from  birth  to  maturity  the  incre- 
ment is  only  twenty-fold.  In  absolute  mass,  however,  95  per  cent  of  the 
final  weight  is  acquired  after  birth.  The  ratio  of  increase  during  each 
fetal  month  to  the  weight  at  the  beginning  of  that  month  is  shown  in  the 
table  on  p.  68. 

Growth  in  Length. — Growth  in  length  and  in  weight  have  certain 
features  in  common,  although  the  relative  increase  in  length  is  obviously 
smaller  since  weight  is  a three  dimensional  phenomenon.  The  increase 
in  the  second  fetal  month  is  ten-fold  but  thereafter  the  relative  rate  of 
growth  gradually  declines.  The  data  of  prenatal  growth  are  given  in  the 
table  on  p.  68.  The  total  postnatal  increment  is  3.3  times.  During  the 
first  six  months  after  birth,  length  increases  30  per  cent;  in  the  first  year, 
50  per  cent.  Throughout  the  most  of  childhood  the  linear  increase  is  very 
slow,  but  at  the  prepubertal  period  there  is  an  acceleration;  as  with  weight, 
this  is  begun  and  ended  earlier  in  girls  than  in  boys.  Growth  is  complete 
at  about  18  years  in  females  and  soon  after  20  in  males.  The  body  is 
heaviest  in  proportion  to  its  length  during  late  fetal  life  and  early  infancy. 
From  the  middle  of  the  first  year  until  after  puberty  there  is  a decline  in 
relative  weight.  Thereafter  there  is  an  increase  in  relative  mass  which 
may  continue  throughout  life.  During  infancy  and  childhood  girls  are 
relative^  lighter  than  boys,  but  after  puberty  the  reverse  is  true. 

Growth  of  Organ  Systems. — The  skeleton  grows  rather  slowly  until 
the  ninth  and  tenth  fetal  months,  when  it  shows  an  acceleration.  At 
birth,  it  constitutes  from  13  to  20  per  cent  of  the  body  weight.  Postnatal 
growth  apparently  parallels  that  of  the  body  as  a whole  and  shows  neither 
relative  loss  nor  gain.  The  musculature  likewise  grows  slowly  at  first,  but 
forms  about  2 5 per  cent  of  the  weight  of  the  newborn  and  40  to  45  per  cent 
of  the  adult.  The  central  nervous  system,  on  the  contrary,  is  relatively 
huge  in  the  young  embryo.  It  decreases  from  about  25  per  cent  in  the 
second  month  to  about  15  per  cent  at  birth  and  2 to  2.5  per  cent  in  the 
adult.  Incomplete  data  on  the  peripheral  nervous  system  and  skin  indicate 
a considerable  reduction  in  relative  weight  during  the  postnatal  years. 
As  a whole,  the  visceral  group  decreases  slowly  and  steadily  in  relative 
weight  after  the  first  two  embryonic  months.  In  the  second  month  they 
comprise  about  15  per  cent  of  the  body  weight,  about  9 per  cent  at  birth, 
and  from  5 to  7 per  cent  in  the  adult. 

Growth  of  the  Organs. — Although  the  general  course  of  relative 
growth  in  the  individual  organs  follows  that  of  the  visceral  group,  each 


84 


AGE,  BODY  FORM  AND  GROWTH  CHANGES 


has  its  characteristic  curve.  Each  usually  increases  more  or  less  rapidly 
to  a maximum  relative  size  and  then  decreases  in  relative  size  through 
the  subsequent  prenatal  and  postnatal  periods. 

During  fetal  life  the  curves  of  absolute  growth  are  quite  similar. 
The  various  organs  have  an  initial  period  of  slow  increase,  followed  after 
the  fifth  month  by  a terminal  phase  of  rapid  growth.  However,  this 
uniformity  disappears  at  birth,  and  most  of  the  organs  can  be  arranged  in 
four  main  divisions.  The  splanchnic  group  includes  the  digestive,  respira- 


PiG.  70. — Chart  showing  the  course  of  growth  in  the  various  organ  groups  (after  Scammon). 

Growth  is  calculated  in  per  cent  of  adult  weight. 

tory,  and  urinary  organs,  and  the  heart,  thyroid,  and  spleen.  The  nervoiis 
group  comprises  the  brain,  cord,  and  eyeballs.  The  genital  group  excludes 
the  ovary  and  uterus  which  have  special  curves.  The  lymphoid  group 
includes  all  but  the  spleen.  Fig.  70  shows  these  relations  graphically 
from  embryo  to  adult. 

Anomalies. — Giants  and  dwarfs  may  be  of  monstrous  size  when  born  at  full  term,  or 
the  acceleration  or  slowing  may  be  secondary  at  some  later  period.  This  abnormal  size  is 
sometimes  unilateral  or  even  confined  to  specific  parts  of  the  body. 


PART  11.  ORGANOGENESIS 
ENTODERMAL  DERIVATIVES 
CHAPTER  V 
THE  DIGESTIVE  SYSTEM 

The  entoderm  of  the  embryonic  disc  is  at  first  directly  continuous 
with  the  entodermal  lining  of  the  yolk  sac,  and  merely  forms  a roof  to 
that  organ  (Fig.  40  C).  As  the  embryo  grows  and  expands,  while  its 
connection  with  the  yolk  sac  lags  in  development,  the  entoderm  neces- 
sarily takes  the  form  of  a blind  tube  within  the  cylindrical  body.  This 
extends  first  into  the  head  region  as  the  fore-gut  (Figs.  40  C and  43),  then 
tailward  as  the  hind-gut  (Fig.  44).  The  intermediate  region,  open  ven- 
trally  through  the  narrower  yolk  stalk  into  the  yolk  sac,  is  sometimes 
termed  the  mid-gut  (Fig.  71),  but  its  existence  is  brief  for  the  yolk  stalk 
loses  its  connection  with  the  gut  during  the  sixth  week. 

At  each  end,  the  gut  comes  into  direct  contact  ventrally  with  the 
ectoderm.  The  plates  thus  formed  are  the  pharyngeal  and  cloacal  mem- 
branes (Fig.  71).  The  pharyngeal  membrane  forms  the  floor  of  a depres- 
sion known  as  the  oral  fossa,  or  stomodeum ; this  fossa  is  bounded  by  the 
fronto-nasal,  maxillary,  and  mandibular  processes  (Figs.  61  and  62).  At 
the  beginning  of  the  fifth  week  (2.5  to  3 mm.  embryos),  the  pharyngeal 
membrane  ruptures  and  the  oral  fossa  and  fore-gut  become  continuous. 
The  oral  fossa  develops  into  the  front  part  of  the  mouth  cavity,  which  is 
therefore  ectodermal.  The  remainder  of  the  mouth  cavity,  the  respira- 
tory tract,  and  the  alimentary  canal  to  a point  well  along  the  small  intes- 
tine are  all  derived  from  the  entodermal  fore-gut. 

The  caudal  end  of  the  entodermal  tube  comprises  the  cloaca,  which 
soon  receives  the  allantoic,  urinary,  and  genital  ducts  (Figs.  87,  91  and  94). 
The  cloaca  promptly  begins  to  subdivide  into  a dorsal  rectum  and  a ventral 
urogenital  sinus  (Figs.  139  to  142).  At  the  same  time,  the  cloacal  mem- 
brane is  separatedinto  anal  and  urogenitalmembranes  (Figs.  71,  95  and  96). 
The  anal  membrane  ruptures  at  about  the  ninth  week,  and  an  external 
depression,  the  proctodeum,  therefore  becomes  continuous  with  the  hind- 
gut  (Figs.  96  and  142).  It  constitutes  the  anal  canal,  which,  like  the  front 
part  of  the  mouth  cavity,  is  lined  with  ectoderm.  The  hind-gut  itself 
forms  some  of  the  small  intestine,  the  colon,  and  the  rest  of  the  rectum 

85 


86 


THE  DIGESTIVE  SYSTEM 


(Figs.  93  to  96).  It  will  be  noticed  that  the  primitive  entodermal  tube 
extends  a little  beyond  the  cloacal  membrane  (Figs.  71,  91  and  139); 
this  tail-gut,  or  postanal  gut,  soon  dwindles  and  disappears. 

The  entoderm  forms  only  the  epithelial  lining  of  these  organs.  AU 
other  coats  develop  from  the  investing  splanchnic  mesoderm.  The 
original  low  epithelium  of  the  gut  differentiates  into  the  several  types  of 
simple  epithelium  formed  in  the  digestive  and  respiratory  systems,  as  well 
as  into  the  pseudostratified  and  stratified  forms.  The  various  glands  are 
primarily  epithelial  outgrowths. 


Pharynx 

Pharyngeal 

}ncnibrane 

Pericardial 

cavity 

Pore-gut 

Hepatic 

diverticulum 


Yolk  stalk 


Hind-gut 

Cloacal 

membrane 

Allantois 

Cloaca 


Yolk  stalk 


Allantois 


Cloacal 

membrane 


Hind-gut 


Pharynx 
Pharyngeal 
membrane 
T hyroid 
gland 
Pericardial 
cavity 


Pore-gut 


Hepatic 

diverticulum 


B 

Fig.  71. — Diagrams  showing  the  human  alimentary  canal  in  median  sagittal  section.  X 35. 
A,  2 mm.  (modified  after  His):  B,  2.5  mm.  (after  Thompson). 


It  is  impossible  to  determine  the  exact  junction  of  ectoderm  and 
entoderm  in  the  mouth,  but  in  general  the  roof  and  peripheral  portions 
are  ectodermal.  The  salivary  glands  are  considered  to  be  from  ectoderm, 
as  are  the  enamel  of  the  teeth,  a portion  of  the  tongue  epithelium,  and 
much  of  the  lining  of  the  nose  and  palate.  Although  these  structures 
do  not  strictly  belong  with  entodermal  derivatives,  it  is  simplest  to  consider 
them  with  the  systems  of  which  they  are  integral  parts. 


THE  MOUTH 


87 


THE  MOUTH 

Lips  and  Cheeks. — During  the  fifth  week,  these  separate  from  the 
jaws  proper  by  the  ingrowth  of  epithelial  plates  which  promptly  begin 
to  thin  and  form  the  vestibule  (Figs.  74,  76  and  79). 

The  Palate. — -The  roof  of  the  original  mouth  cavity  is  the  base  of 
the  skull.  When  the  membranes  which  separate  the  olfactory  pits  from 
the  mouth  rupture,  their  orifices,  the  primitive  choance,  also  open  into  the 
common  oral  cavity.  The  nasal  passages  next  become  separate  by 
partitioning  off  a portion  of  the  mouth  cavity  and  adding  it  to  their 
original  extent.  They  then  communicate  with  the  pharynx  by  the 


Nasal  septum 


1 ongue 

Lateral  palatine  process 


A 


Fig.  72. — Sections  through  the  jaws  of  pig  embryos,  to  show  the  development  of  the  palate 
(Prentiss).  X 8.  A,  22  mm.;  B,  34  mm. 


secondary,  definitive  choance.  The  horizontal  septum  which  thus  divides 
mouth  from  nasal  ^passage  is  the  palate.  The  details  of  its  formation 
follow ; 

At  first  the  jaws  are  closed  and  the  tongue  extends  up  between  shelf - 
like  folds  of  the  maxilla,  the  lateral  palatine  processes  (Figs.  73  A and  74), 
which  project  downward  (Fig.  72  A).  Soon  the  mandible  drops,  owing  to 
growth  changes,  and  the  tongue  is  withdrawn.  This  allows  the  palatine 
folds  to  bend  upward  to  the  horizontal  plane  (Fig.  75),  approach,  and  fuse 


88 


THE  DIGESTIVE  SYSTEM 


Oral  cavity 


Median  palatine 
process 

Lateral  palatine 
process 

Internal  choance 


Median  palatine 
process 

Raphe  of  lateral 
palatine  process 


Nasal  passage 
Anlage  of  uvula 


A B 

Fig.  73- — Dissections  to  show  the  development  of  the  palate  in  pig  embryos  (Prentiss). 
X 5.  A , The  upper  jaw  and  palatine  processes  of  a 22  mm.  embryo  in  ventral  view;  B,  fusion 
of  the  palatine  processes  in  a 35  mm.  embryo. 


Median  palatine  process 


Processus  globiilaris 

(Median  nasal  process) 


Lateral  palatine  process 


Maxillary  process 


Fig. 


74. — The  roof  of  the  mouth  of  a two-months’  human  embryo, 
groove,  primitive  choanse  and  developing  palate  (after  His).' 


showing  the  labial 
,X  9- 


Nasal  septum 


Fig.  75. — Section  through  the  jaws  of  a 25  mm.  pig  embryo,  to  show  the  change  in  position  of 
one  palatine  process  due  to  unequal  growth  (Prentiss). 


THE  MOUTH 


89 


(Fig.  72  B).  The  shift  of  the  lateral  palatine  processes  is  an  active  bend- 
ing, due  to  cellular  proliferation  on  their  under  sides  (Fig.  75).  The 
union  of  the  halves  of  the  palate  begins  about  the  end  of  the  second  month 
and  progresses  backward  toward  the  pharynx  (Fig.  73  B).  Coincidently, 
bone  appears  in  the  front  part  and  forms  the  hard  palate;  more  caudad, 
ossification  fails,  and  this  region  constitutes  the  soft  palate  and  its  free 
apex  the  uvida.  The  unfused,  backward  prolongations  of  the  palatine 
folds  give  rise  to  the  pharyngo-palatine  arches,  which  delimit  oral 
cavity  from  pharynx.  The  palate  shows  a median  seam,  and,  for  a 
time,  the  uvula  is  notched;  both  are  indicative  of  the  mode  of  origin. 

The  median  nasal  lobes  of  the  original  fronto-nasal  process  also 
develop  median  palatine  processes,  so-called,  which  do  not  contribute  to 
the  palate  but  form  the  premaxillary  portion  of  the  upper  jaw  (Figs. 
73  and  74).  Fusion  with  the  palate  is  incomplete  and  in  the  midplane 
there  is  a gap,  the  incisive  foramen,  flanked  by  the  incisive  canals  (of 
Stenson).  These  become  covered  with  mucous  membrane,  although 
they  sometimes  are  patent  at  birth. 


Fig.  76. — Early  stages  in  the  development  of  the  teeth  (Rose).  .4,  Seven  weeks  (X  90);  B, 

nine  weeks  (X  45)- 

Anomalies. — The  lateral  palatine  processes  occasionally  fail  to  unite  in  the  middle 
line,  producing  a defect  known  as  deft  palate.  The  extent  of  the  defect  varies  considerably, 
in  some  cases  involving  only  the  soft  palate,  while  in  other  cases  both  soft  and  hard  palate 
are  cleft.  It  may  be  associated  also  with  hare  lip. 

The  Teeth. — The  teeth  have  a double  origin.  The  enamel  is  from 
ectoderm ; the  dentine,  pulp,  and  cement  are  mesodermal. 

The  Enamel  Organ. — When  the  labial  groove  is  forming  in  embryos 
of  six  weeks,  a horizontal  shelf  develops  from  it  and  extends  backward 
into  the  substance  of  the  jaw  (Fig.  76).  This  curved  dental  ridge,  or 
lamina,  is  parallel  with  the  adjacent  labial  groove  and  lies  mesial  to  it 
(Fig.  82).  At  intervals  a series  of  thickenings  develop,  the  anlages  of 
the  enamel  organs;  these  will  form  enamel  and  serve  as  the  moulds  of  the 
future  teeth  (Figs.  76  B and  77).  Early  in  the  third  month  the  ventral 
side  of  each  enamel  organ  becomes  concave,  like  an  inverted  cup,  and  the 


90 


THE  DIGESTIVE  SYSTEM 


concavity  is  occupied  by  dense  mesenchymal  tissue,  the  dental  papilla, 
which  will  differentiate  into  dentine  and  pulp  (Figs.  77  and  78).  An 
enamel  organ  and  its  associated  dental  papilla  is  the  basis  of  each  tooth 


Oral  epithelium  Enamel  organs 

'\ 


1/ 

Dental  lamina 

Papilke 

A 


Free  edge  of 
the  dental 

\l/  ^ lamina 

Enamel  organs  enamel  organs 

C D 


Fig.  77. — Models  of  the  early  development  of  three  teeth,  one  in  section  (Lewis  and  StohrL 


(Fig.  79).  Ten  such  anlages  of  the  decidual,  or  milk  teeth,  are  present  in 
each  jaw  (Fig.  82).  Their  connection  with  the  dental  ridge  is  eventually 
lost. 


Dental  tamina 


Fig.  78. — Section  through  an  upper  incisor  from  a fetus  of  three  months  (Prentiss).  X 70. 

The  compact  internal  cells  of  the  enamel  organ  transform  into  a 
reticulum  resembling  mesenchyme,  termed  the  enamel  pulp  (Fig.  78). 
The  outer  enamel  cells,  at  first  cuboidal,  flatten  out  as  a fibrous  layer. 
Neither  of  these  components  contributes  to  tooth  formation.  The  inner 


THE  MOUTH 


91 


enamel  cells  line  the  cup-shaped  concavity  of  the  enamel  organ.  Over 
the  crown  of  the  tooth  these  cells  are  designated  amelohlasis,  for  they 
become  columnar  and  produce  the  enamel  layer  along  their  basal  ends 


Fig.  79. — Parasagittal  section  through  the  mandible  and  tongue  of  a three-months’  fetus, 
showing  the  relations  of  the  first  incisor  anlage  (Prentiss).  X 14. 

(Fig.  80).  The  enamel  is  laid  down  first  as  an  uncalcified,  fibrillar  layer 
which  then  calcifies  in  the  form  of  enamel  prisms,  one  for  each  ameloblast. 
The  enamel  is  deposited  first  at  the  apex  of  the  crown  and  then  downward 


Fig.  80. — Section  through  a portion  of  the  crown  of  a developing  tooth,  showing  the  various 

layers  (Tourneux  in  Heisler). 

toward  the  root.  The  enamel  cells  about  the  future  root  of  the  tooth 
remain  cuboidal  or  low  columnar  in  form,  come  into  contact  with  the  outer 
enamel  cells,  and  the  two  layers  constitute  the  epithelial  sheath  of  the 
root  (Fig.  81);  it  does  not  produce  enamel  prisms. 


92 


THE  DIGESTR'E  SYSTEM 


The  Dental  Papilla  — At  the  end  of  the  fourth  month,  the  outermost 
cells  of  the  dental  i)apilla  arrange  themselves  as  a definite  layer  of  columnar 
epithelium.  Since  they  produce  the  dentine,  or  dental  bone,  these  cells 
are  known  as  odontoblasts  (Fig.  8i).  When  the  dentine  layer  is  deposited, 


Denial  sac 


Enamel  pulp 


Inner  enamel  cells 
{amelohlasts) 


Enamel  j 


V . Odontohlasls 


enamel  cells 


1 .Dentine 


Epithelial  sheath 


Outer  la 


Inner  layer 


Dental  papilla  (future  pulp) 

Blood  vessel 


Bony  trabecula  of  jaw  — 


Fig.  8i. — Longitudinal  section  of  a decidual  tooth  of  a newborn  dog.  X 42.  Above  the 
enamel,  on  either  side,  are  artificial  shrinkage  spaces  (Lewis  and  Stohr). 


the  odontoblast  cells  remain  internal  to  it,  but  branched  processes  from 
them  (the  dentinal  fibers  of  Tomes)  extend  into  the  dentine  and  occupy  the 
dental  canalicnli  (Fig.  80).  Internal  to  the  odontoblasts,  the  remaining 
mesenchymal  cells  differentiate  into  the  dental  pulp,  popularly  known 


THE  MOUTH 


93 


as  the  ‘nerve’  of  the  tooth.  This  is  composed  of  a framework  of  reticular 
tissue  in  which  are  found  blood  vessels,  lym.phatics,  and  nerve  fibers. 
The  odontoblast  layer  persists  throughout  life  and  intermittently  lays 
down  dentine,  so  that  eventually  the  root  canal  may  be  obliterated. 

The  Dental  Sac. — The  mesenchymal  tissue  surrounding  the  anlage  of 
the  tooth  gives  rise  to  a dense  outer  layer  and  a more  open  inner  layer 
of  fibrous  connective  tissue.  These  form  the  dental  sac  (Fig.  8i).  Over 
the  root  of  the  tooth  a layer  of  osteoblasts,  or  bone  forming  cells,  develops, 
and  when  the  epithelial  sheath  of  the  enamel  organ  disintegrates,  they 
deposit  about  the  dentine  an  investment  of  specialized  bone,  known  as 
the  cement.  Cementum  contains  typical  bone  cells  but  no  Haversian 
systems.  As  the  tooth  grows  and  fills  its  alveolar  socket,  the  dental  sac 


Milk 
molar  I 


Aboral 
prolonga- 
tion oj 
dental 
lamina 

Fig.  82. — Dental  lamina  and  anlages  of  the  upper  milk  teeth  in  a three-months’  fetus  (Rose). 

becomes  a thin,  vascular  layer,  the  peridental  membrane.  This  has  fibrous 
attachments  to  both  the  alveolar  bone  and  the  cement  and  holds  the  tooth 
in  place. 

Eruption. — ^When  the  crown  of  the  tooth  is  fully  developed  the  enamel 
organ  disintegrates,  and,  as  the  root  continues  to  grow,  the  crown 
approaches  the  surface  and  breaks  through  the  gum.  The  periods  of 
eruption  of  the  various  milk,  or  decidual  teeth  vary  with  race,  climate,  and 
nutritive  conditions  Usually  they  are  cut  in  the  following  sequence: 


IVIedian  Incisors sixth  to  eighth  month. 

Lateral  Incisors eighth  to  twelfth  month. 

First  INIolars twelfth  to  sixteenth  month. 

Canines seventeenth  to  twentieth  month. 

Second  Molars twentieth  to  thirty-sixth  month. 


The  permanent  teeth  develop  precisely  like  the  temporary  set.  The 
anlages  of  those  permanent  teeth  which  correspond  to  the  milk  dentition 


94 


THE  DIGESTIVE  SYSTEM 


arise  in  another  series  along  the  free  edge  of  the  dental  lamina  (Fig.  77  H) 
and  come  to  lie  mesad  of  the  decidual  teeth  (Fig.  83).  In  addition,  three 
permanent  molars  are  developed  on  each  side,  both  above  and  below,  from 
a backward  or  alioral  extension  of  the  dental  lamina,  entirely  free  from  the 
oral  epithelium  (Fig.  82).  The  anlages  of  the  first  permanent  molars 
appear  at  the  end  of  the  fourth  month,  those  of  the  second  molars  at  six 
weeks  after  birth,  while  the  anlages  of  the  third  permanent  molars,  or 
wisdom  teeth,  are  not  found  until  the  fifth  year.  The  permanent  dentition 
of  thirty-two  teeth  is  then  complete. 

Before  the  permanent  teeth  begin  to  erupt,  the  roots  of  the  milk  teeth 
undergo  partial  resorption,  their  dental  pulp  dies,  and  they  are  eventually 


Permanent  second  molar 


Deciduous  molars 
Mandibular  canal 

Permanent  first  molat 


Permanent  premolars 

Permanent  canine 


Mental  foramen 
Perma7ient  incisors 


Fig  83. — Skull  of  a live-year-old  child,  showing  the  positions  of  the  decidual  and  permanent 

teeth  (Sobotta-McMurrich). 


shed.  Toward  the  sixth  year,  before  the  loss  of  the  decidual  teeth  begins, 
each  jaw  may  contain  twenty-six  teeth  (Fig.  83).  The  permanent  teeth 
are  cut  as  follows : 


First  Molars 

Median  Incisors 

Lateral  Incisors 

First  Premolars 

Second  Premolars 

Canines  I 

Second  Molars ) 

Third  Molars  (Wisdom  Teeth) 


seventh  year, 
eighth  year, 
ninth  year, 
tenth  year, 
eleventh  year. 

thirteenth  to  fourteenth  year, 
seventeenth  to  fortieth  year. 


The  teeth  of  vertebrates  are  homologues  of  the  placoid  scales  of  elasmobranch  fishes 
(sharks  and  skates).  The  teeth  of  the  shark  resemble  enlarged  scales,  and  many  genera- 


THE  MOUTH 


95 


tions  are  produced  in  the  adult  fish.  In  some  mammalian  embryos,  three,  or  even  four, 
dentitions  are  present.  The  primitive  teeth  of  mammals  were  of  the  canine  type,  and  from 
this  conical  tooth  the  incisors  and  molars  have  arisen.  Just  how  the  cusped  tooth  differ- 
entiated— whether  by  the  fusion  of  originally  separate  units,  or  by  the  development  of 
cusps  on  a single  primitive  tooth — is  debated. 


Arytenoid  ridge 


Lateral  lingual  anlage 
Tiihercidnm  impar 


Epiglottis 


Branchial  arch 


Branchial  arch 
Branchial  arch 


3 

4 


Branchial  arch  i 
Tiihercidian  impar 

Branchial  arch  2 

Branchial  arch  j 
Branchial  arch  4 

Arytenoid  ridge 


Lateral  lingual  anlage 

Copula 

Epiglottis 

Glottis 


B 


Branchial  arch  i~ 
Lateral  lingual  anlage  - 


■ Tnberadum  impar 

- Branchial  arch  2 

Epiglottis 

Glottis 


Fig.  84. — Dissections  showing  the  development  of  the  tongue  in  pig  embryos  (Prentiss). 
X 12.  A,  7 mm.;  B,  9 mm.;  C,  13  mm. 


Anomalies. — Dental  anomalies  are  frequent.  They  may  consist  in  the  congenital 
absence  of  some  or  all  of  the  teeth,  or  in  the  production  of  more  than  the  normal  number. 
Defective  teeth  are  frequently  associated  with  hare  lip.  Cases  have  been  noted  in  which, 
owing  to  a defect  of  the  enamel  organ,  the  enamel  was  wanting.  Third  dentitions  have 
been  recorded,  and  occasionally  fourth  molars  are  developed  behind  the  wisdom  teeth. 


96 


THE  DIGESTIVE  SYSTEM 


The  Tongue.  -The  tongue  develo])s  as  two  distinct  portions,  the 
body  and  the  root,  separated  from  each  other  by  a V-sha]ied  groove,  the 
sulcus  tcnninalis  (Fig.  85  B).  In  both  human  and  pig  embryos,  the  body  of 
the  tongue  is  represented  by  three  anlages  that  appear  in  front  of  the 
second  branchial  arches.  These  are  the  median,  somewhat  triangular 
iuhcrciilitm  iiupar,  and  the  paired  lateral  swellings  of  the  first,  or  mandibu- 
lar arches — all  of  which  are  present  in  human  embryos  of  5 mm.  (Figs.  84  A 
and  85  .4).  At  this  stage,  a median  ventral  elevation,  formed  by  the  union 
of  the  second  branchial  arches,  constitutes  the  copula.  This,  with  the 
])ortions  of  the  second  arches  lateral  to  it,  forms  later  the  root  of  the  tongue. 
Between  it  and  the  tuberculum  impar  is  the  point  of  evagination  of  the 
thyroid  gland,  represented  in  the  adult  by  the  foramen  cecum  (Fig.  85). 
I'he  copula  also  connects  the  tuberculum  impar  with  a rounded  prominence 
that  is  developed  in  the  midventral  line  from  the  bases  of  the  third  and 


Fig.  85. — Stages  in  the  development  of  the  human  tongue  (adapted).  A,  6 mm.;  B,  15 
mm.  Contributions  from  the  first  three  branchial  arches  are  indicated  respectively  by  parallel 
lines,  dots  and  crosses;  the  tuberculum  impar  is  marked  by  circles. 


fourth  branchial  arches.  This  is  the  anlage  of  the  epiglottis  (Figs.  84 
and  85). 

In  later  stages  (Figs.  84  B,  C and  85  B),  the  lateral  mandibular  anlages, 
bounded  laterally  by  the  alveolo-lingual  grooves,  increase  rapidly  in  size 
and  fuse  with  the  tuberculum  impar,  which  lags  behind  in  development, 
and,  according  to  recent  investigators,  atrophies  completely.  The  epiglot- 
tis enlarges  and  becomes  concave  on  its  ventral  surface.  Caudad,  and  in 
early  stages  continuous  with  it,  are  two  thick,  rounded  folds,  the  arytenoid 
ridges.  Between  these  is  the  slit-like  glottis,  leading  into  the  larynx. 

In  fetuses  of  ii  weeks,  the  fungiform  and  filiform  papilla;  may  be 
distinguished  as  elevations.  Taste  buds  appear  in  the  fungiform  papillae 
at  14  weeks  and  are  much  more  numerous  in  the  fetus  than  in  the  adult. 


THE  MOUTH 


97 


The  vallate  papilla:  develop  on  a V-shaped  epithelial  ridge  whose  apex 
corresponds  to  the  site  of  the  thyroid  evagination  (Fig.  85  B).  After  the 
thirteenth  week,  circular  epithelial  downgrowths  occur  at  intervals  along 
the  ridges  and  take  the  form  of  inverted  and  hollow  truncated  cones  (Fig. 
86  A).  During  the  fourth  month  circular  clefts  appears  in  the  epithelial 
downgrowths,  thus  separating  the  walls  of  the  vallate  papillae  from  the 
surrounding  epithelium  and  forming  the  trench  from  which  this  type  of 
papilla  derives  its  name  (Fig.  86  B).  At  the  same  time,  lateral  outgrowths 
arise  from  the  bases  of  the  epithelial  cones,  hollow  out  and  form  the 
ducts  and  glands  of  Ebner  (Fig.  86  C).  The  taste  buds  of  the  vallate 
papillae  also  are  formed  early,  appearing  in  embryos  of  three  months. 
Foliate  papilla:  probably  develop  at  about  six  months. 

The  foregoing  account  applies  to  the  early  origin  of  the  mucous  membrane  alone.  The 
musculature  of  the  tongue  is  supplied  chiefly  by  the  hypoglossal  nerve,  and  both  nerve  and 
muscles  belong  historically  to  the  postbranchial  region.  If  not  in  the  development  of  each 
present-day  embryo,  at  least  in  the  past  the  musculature  has  migrated  cephalad  and  in- 
vaded the  branchial  region  beneath  the  mucous  membrane  (cf.  p.  229).  At  the  same  time, 
the  tongue  may  be  said  to  extend  caudad  until  its  root  is  covered  by  the  epithelium  of  the 


Fig.  86. — Diagrams  showing  the  development  of  the  vallate  papillae  of  the  tongue  (Graberg 
in  McMurrich).  a,  Valley;  b,  von  Ebner's  gland. 

third  and  fourth  branchial  arches.  This  is  shown  by  the  fact  that  the  sensory  portions  of 
the  nn.  trigeminus  a.nA  facialis,  the  nerves  of  the  first  and  second  arches,  supply  the  body  of 
the  tongue,  while  the  nn.  glossopharyngeus  and  vagus,  the  nerves  of  the  third  and  fourth 
arches,  supply  chiefly  the  root. 

Anomalies. — Faulty  development  or  incomplete  fusion  of  the  several  anlages  causes 
variable  degrees  of  absence  or  bifurcation  of  the  tongue. 

The  Salivary  Glands. — The  glands  of  the  mouth  are  all  regarded  as 
derivatives  of  the  ectodermal  epithelium.  They  complete  their  differentia- 
tion only  after  birth. 

The  parotid  is  the  first  to  develop.  Its  anlage  has  been  observed  in 
8 mm.  embryos,  near  the  angle  of  the  mouth,  as  a keel-like  flange  in  the 
floor  of  the  groove  which  divides  cheek  from  jaw.  The  flange  elongates, 
and,  in  embryos  of  seven  weeks,  separates  from  the  parent  epithelium, 
forming  a tubular  structure  that  opens  into  the  mouth  cavit\"  near  the 
front  end  of  the  original  furrow.  The  tube  grows  back  into  the  region 


g8 


THE  DIGESTR'E  SYSTEM 


of  the  external  ear,  branches,  and  forms  the  main  body  of  the  gland  in  this 
region,  while  the  stem  portion  of  the  tube  becomes  the  parotid  duct. 
Acinus  cells  are  present  at  five  months. 

The  siibmaxillary  gland  arises  at  n mm.  as  an  epithelial  ridge  in  the 
groove  between  the  jaw  and  the  tongue,  its  cephalic  end  located  near  the 
frenulum.  The  caudal  end  of  the  ridge  soon  begins  to  separate  from 
the  epithelium  and  extend  backward  and  ventrad  into  the  submaxillary 


Fig.  87. — Diagrammatic  ventral  view  of  pharynx,  digestive  tube  and  mesonephroi  of  a 
4 to  5 mm.  embryo  (adapted  by  Prentiss).  X about  30.  The  liver  and  yolk  sac  are  cut 
away.  The  tubules  of  the  right  mesonephros  are  shown  diagrammatically. 


region,  where  it  enlarges  and  branches  to  form  the  gland  proper ; its  cephalic, 
unbranched  portion,  persisting  as  the  duct,  soon  hollows  out  (Fig.  79). 

The  sublingual  gland  appears  by  the  eighth  week  as  several  solid 
evaginations  of  epithelium  from  the  jaw-tongue  groove  (Fig.  79).  This 
group,  usually  regarded  as  a sublingual  gland,  really  consists  of  the  sublin- 
gual proper,  with  its  ductus  major,  and  of  about  ten  equivalent  alveolo- 
lingual  glands.  Mucin  cells  have  appeared  by  the  sixteenth  week. 


THE  PHARYNX 


99 


THE  PHARYNX 


Pharyngeal  Pouches. — There  are  developed  early  from  the  lateral 
wall  of  the  entodermal  pharynx  paired  outpocketings  which  are  formed 
in  succession  cephalo-caudad.  In  4 to  5 mm.  embryms,  five  pairs  of  such 
pharyngeal  {branchial)  pouches  are  present,  the  fifth  pair  being  rudimen- 
tary (Figs.  87  and  91).  Meanwhile,  the  pharynx  has  flattened  and 
broadened,  so  that  it  is  triangular  in  ventral  view  (Figs.  87  and  88). 

From  each  pharyngeal  pouch  develop  small  dorsal  and  large  ventral 
diverticula.  All  five  pouches  come  into  contact  with  the  ectoderm 


Branchial  duct  2 Parathyroid  of  jd  pouch 


Branchial  groove  1 


~ervical  sinus 


Pharyngeal  pouch  i 
Pharyngeal  pouch  2 
Pharyngeal  pouch  j 


Cervical  vesicle 
Thymus  anlage 

Parathyroid  tf  ph  pouch 


Pharyngeal  pouch  4 


Trachea 


Stomach 
Dorsal  pancreas 


Pharyngeal  pouch  5 
Esophagus 


.4 pical  bud  of  right  lung 


Gall  bladder 
Duodenum 


Fig.  88. — Reconstruction  of  the  pharynx  and  fore-gut  of  a 12  mm.  human  embryo,  seen  in 
dorsal  view  (Hammar-Prentiss).  The  ectodermal  structures  are  stippled. 


of  corresponding  branchial  grooves,  fuse  with  it,  and  form  the  closing  plates. 
Although  the  closing  plates  become  perforate  in  human  embryos  only 
occasionally,  these  pouches  and  grooves,  nevertheless,  are  homologous  to 
the  functional  branchial  clefts  of  fishes  and  tailed  amphibia.  The  first 
and  second  pharytigeal  pouches  soon  connect  with  the  pharyngeal  cavity 
through  wide  common  openings.  The  third  and  fourth  pouches  grow 
laterad  and  their  diverticula  communicate  with  the  pharynx  through 


lOO 


THE  DIGESTIVE  SYSTEM 


narrow  ducts  in  lo  to  12  mm.  embryos  (Fig.  88).  When  the  cervical  sinus 
(p.  77)  is  formed,  the  ectoderm  of  the  second,  third,  and  fourth  branchial 
clefts  is  drawn  out  to  produce  the  transient  branchial  and  cervical  ducts  and 
the  cervical  vesicle.  These  are  fused  at  the  closing  plates  with  the  entoderm 
of  the  corresponding  pharyngeal  pouches. 

The  fate  of  the  entodermal  pouches  is  varied  and  spectacular. 
Although  they  do  not  continue  as  parts  of  the  digestive  apparatus,  their 
embryonic  relations  justify  their  inclusion  in  the  present  section.  The 
first  differentiates  into  the  tympanic  cavity  of  the  middle  ear  and  into 
the  auditory  (Eustachian)  tube.  The  second  becomes  the  palatine  tonsil 
in  part.  The  third,  fourth,  and  fifth  pouches  give  rise  to  a series  of  ductless 
glands:  the  thymus,  parathyroids,  and  the  ultimobranchial  bodies. 

The  Tonsils. — By  the  growth  and  lateral  expansion  of  the  pharynx, 
the  second  pouch  is  absorbed  into  the  pharyngeal  wall,  its  dorsal  angle 
alone  persisting,  to  be  transformed  into  the  tonsillar  and  su proton sillar 
fossae.  Crypts  arise  at  the  end  of  the  third  month  by  the  hollowing  of 
solid  epithelial  ingrowths,  whereas  a mound  of  mesodermal  lymphoid 
tissue  hrst  presses  against  the  epithelium  at  the  middle  of  the  fourth  month. 
This  association  constitutes  the  palatine  tonsil. 

A .subepithelial  infiltration  of  lymphocytes  during  the  sixth  month  gives  rise  to  the 
median  pharyngeal  tonsil,  which  like  the  lingual  tonsil  is  not  of  pharyngeal  pouch  origin. 
Immediately  caudad  is  a recess,  the  pharyngeal  bursa,  formed  by  a protracted  connection 
of  the  epithelium  with  the  notochord  (Huber).  It  bears  no  relation  to  the  original  blind 
termination  of  the  fore-gut  known  as  Seesel’s  pouch.  According  to  Hammar,  the  lateral 
pharyngeal  recess  (of  Rosenmfiller)  is  not  a persistent  portion  of  the  second  pouch,  as 
His  asserted. 

The  Thymus. — The  thymus  anlages  appear  in  10  mm.  embryos  as 
ventral  and  medial  prolongations  of  the  third  pair  of  pouches  (Figs.  88 
and  89).  The  ducts  connecting  the  diverticula  with  the  pharynx  soon 
disappear  so  that  the  anlages  are  set  free.  At  first,  they  are  hollow  tubes 
which  soon  lose  their  cavities  and  migrate  caudally  into  the  thorax, 
usually  passing  ventral  to  the  left  innominate  vein.  Their  upper  ends 
become  attentuate  and  atrophy,  but  may  persist  as  accessory  thymus 
lobes.  The  enlarged  lower  ends  of  the  anlages  form  the  body  of  the 
gland,  which  is  thus  a paired  structure  (Fig.  90).  At  1 1 weeks  the  thymus 
still  contains  solid  cords  and  small  closed  vesicles  of  entodermal  cells. 
From  this  stage  on,  the  gland  becomes  more  and  more  lymphoid  in  charac- 
ter. Its  final  position  is  in  the  thorax,  dorsal  to  the  upper  end  of  the 
sternum.  It  grows  under  normal  conditions  until  puberty,  after  which 
involution  begins.  This  process  proceeds  slowly  in  healthy  individuals, 
rapidly  in  case  of  disease.  True  atrophy  of  the  parenchyma  enters  at 
about  the  fiftieth  year. 


THE  PHARYNX 


lOI 


The  ventral  diverticulum  of  the  fourth  pouch  is  a rudimentary  thymic 
anlage  which  usually  atrophies. 

It  is  now  generally  believed  that  the  entodermal  epithelium  of  the  thymus  is 
converted  into  reticular  tissue  and  thymic  corpuscles.  The  latter  are  the  atrophic  and 
hyalinized  remains  of  embryonic  tubules  and  cords  (Marine,  1915).  The  lymphoid 


Fig.  89. — Diagram  of  the  pharynx  and  its  derivatives  (adapted  by  Prentiss).  I-V,  first  to 

fifth  pharjmgeal  pouches. 

cells  were  regarded  by  Stohr  as  entodermal  in  origin,  but  most  observers  derive  them 
from  the  mesoderm. 

The  Parathyroid  Glands. — Each  dorsal  diverticulum  of  the  third  and 
fourth  pharyngeal  pouches  gives  rise  to  a small  mass  of  epithelial  cells 


Jugular  Carotid  Carotid 

vein  artery  artery 


Fig.  90. — Reconstruction  of  the  thymus,  thyroid  and  parathj-roid  glands  in  a human  embryo 
of  two  months  (Tourneaux  and  Verdun).  X 15. 

termed  a parathyroid  gland  (Fig.  89).  Two  pairs  of  these  bodies  are  thus 
formed,  and,  with  the  atrophy  of  the  ducts  of  the  pharyngeal  pouches, 
they  are  set  free  and  migrate  caudalw^ard.  The}*  eventually  lodge  in  the 
dorsal  surface  of  the  thyroid  gland;  the  pair  from  the  third  pouches  lies, 


102 


THE  DIGESTR’E  SYSTEM 


one  on  each  side,  at  its  caudal  border,  the  ]iair  from  the  fourth  pouches  at 
the  cranial  border  (Fig.  go).  Their  solid  bodies  are  broken  up  into 
masses  and  cords  of  jiolyhedral  entodermal  cells  intermingled  with  blood 
vessels.  In  postfetal  life,  lumina  may  appear  in  the  cell  masses  and  fill 
with  a colloid-like  secretion. 

The  Ultimobranchial  Bodies.  - These  bodies,  also  called  postbranchial, 
are  usually  rated  as  derivatives  of  the  fifth  pharyngeal  pouches  (Fig.  89). 


Melcncephaloii 


Aortic  arches  2-1^,  & 


Notochord, 

Descending  aorta 
Trachea 
Lung  hud 
Esophagus 


Stomach 
Vitelline  vein 

Dorsal  aorta. 


Hind-gut' 


Mescnccphaton  and  cephalic  flexure 
Rathke’s  pouch 
Diencephalon 

Internal  carotid  artery 

Optic  vesicle 

Prosencephalon 

Mouth  cavity 

Pharyngeal  pouches  1~4 

Ventral  aorta 
Atrium  of  heart 
Umbilical  vei7i 
Liver  anlage 
Splanchnic  mesoderm 
Mid-gut 

Entoderm  of  yolk  stalk 
Tail  gut 

Umbilical  artery 
Mesonephric  duct 


Cloaca 

Allatitois 

Fig.  91. — Reconstruction  of  a 4.2  human  embryo  (His- Prentiss).  X 25. 


By  the  atrophy  of  the  ducts  of  the  fourth  pouches  they  are  set  free  and 
migrate  caudad  with  the  parathyroids.  Each  forms  a hollow  vesicle 
which  has  been  erroneously  termed  the  lateral  thyroid.  It  takes  no  part 
in  forming  thyroid  tissue,  but  atrophies.  Kingsbury  (1915)  denies  the 
origin  of  the  ultimobranchial  body  from  any  specific  pouch,  and  asserts 
it  is  “merely  formed  by  a continued  growth  activity  in  the  branchial 
entoderm’’. 


THE  UIGESTTV’E  TUBE 


10.3 

The  Thyroid  Gland. — In  embryos  with  five  to  six  primitive  segments 
(1.4  mm.)  there  appears  in  the  midventral  wall  of  the  pharynx,  between  the 
first  and  second  branchial  arches,  a small  outpocketing,  the  thyroid  milage. 
In  2.5  mm.  embryos  it  has  become  a stalked  vesicle  (Figs.  71  j5  and  87). 
Its  stalk,  the  thyroglossal  duct,  opens  at  the  aboral  border  of  the  tuber- 
culum  impar  of  the  tongue  (Figs.  85  B);  this  spot  is  represented  perma- 
nently by  the  foramen  cecum  (Fig.  g6).  The  duct  soon  atrophies  and  the 
bilobled  gland  anlage  (Fig.  89)  loses  its  lumen  and  breaks  up  into  irregular, 
solid,  anastomosing  plates  of  tissue  as  it  migrates  caudad.  The  thyroid 
assumes  a transverse  position  with  a lobe  on  each  side  of  the  trachea  and 
larynx  (Fig.  90).  In  embryos  of  eight  weeks,  discontinuous  lumina  begin 
to  appear  in  swollen  portions  of  the  plates;  these  represent  the  primitive 
thyroid  follicles.  Colloid  soon  forms. 

Anomalies. — Persistent  portion  of  the  thyroglossal  duct  may  form  cysts  or  even 
fistulce. 

THE  DIGESTIVE  TUBE 

The  several  accessory  coats  of  the  digestive  tube  are  all  derived  from 
splanchnic  mesoderm  wTich  invests  the  entoderm  of  the  primitive  gut. 
In  each  division  of  the  tube  the  circular  muscle  layer  develops  before  the 
longitudinal  layer. 


D,  19  mm. 

The  Esophagus. — The  esophagus  in  4 to  5 mm.  embryos  is  a very  short 
tube,  extending  from  pharynx  to  stomach  (Fig.  91).  As  the  heart  and 
diaphragm  recede  into  the  thorax,  it  grows  rapidly  in  length  (Figs.  95 
and  96).  In  embryos  of  8 mm.  the  esophageal  epithelium  is  composed  of 
two  layers  of  columnar  cells,  but  at  birth  they  number  nine  or  ten.  The 
esophagus  remains  so  broadly  attached  to  the  dorsal  body  wall  that  there 
is  never  a distinct  mesentery  (Fig.  124). 

During  the  eighth  week  vacuoles  appear  in  the  epithelium  and  increase  the  size 
of  the  lumen,  which,  however,  is  at  no  time  occluded.  Glands  begin  to  develop  at 
four  months.  The  circular  muscle  layer  is  indicated  at  six  weeks  but  the  longitudinal 
fibers  do  not  form  a definite  layer  until  ii  weeks. 


104 


THE  DIGESTIVE  SYSTEM 


Anomalies. — There  may  be  atresia.  This  usually  involves  a fistulous  relation  with 
the  trachea;  the  esophagus  is  divided  transversely,  the  trachea  opening  into  the  lower 
segment,  while  the  upper  portion  ends  as  a blind  sac. 

The  Stomach. — The  stomach  appears  in  embryos  of  4 to  5 mm.  as  a 
laterally  flattened,  fusiform  enlargement  of  the  fore-gut,  caudal  to  the  lung 
anlages  (Figs.  93  and  94).  Its  wall  is  composed  of  three  layers:  the 
entodermal  epithelium,  a thick  mesenchymal  layer,  and  the  peritoneal 
mesothelium  (Fig.  114).  The  stomach  is  attached  dorsally  to  the  body 
wall  by  its  mesentery,  the  greater  omentum,  and  ventrally  to  the  liver  by 
the  lesser  omentum  (Fig.  112  B).  The  dorsal  border  of  the  stomach  soon 
bulges  locally  to  form  the  fundus,  and  also  grows  more  rapidly  than  the 
ventral  wall  throughout  its  extent,  thus  producing  the  convex  greater 

Pharynx 


Fig.  93. — Median  sagittal  section  of  a 5 mm.  human  embryo,  to  show  the  digestive  canal 

(Ingalls-Prentiss).  X 14. 

curvature  (Fig.  92).  The  whole  stomach  becomes  curved,  and  its  cranial 
end  is  displaced  to  the  left  by  the  enlarging  liver  (Fig.  88).  This  forms  a 
ventral  concavity,  the  lesser  curvature,  and  produces  the  first  flexure  of  the 
duodenum. 

The  rapid  growth  of  the  gastric  wall  along  the  greater  curvature  also 
causes  the  stomach  to  rotate  about  a long  axis  until  its  greater  curvature, 
or  primitive  dorsal  wall,  lies  to  the  left,  its  lesser  curvature,  or  ventral  wall, 
to  the  right  (Fig.  114).  The  original  right  side  is  now  dorsal,  the  left 
side  ventral  in  position,  and  the  caudal,  or  pyloric  end  of  the  stomach  is 
ventral  and  to  the  right  of  its  cardiac,  or  cephalic  end.  The  whole  organ 
extends  obliquely  across  the  peritoneal  cavity  from  left  to  right. 


THE  DIGESTIVE  TUBE 


105 

These  changes  in  position  progress  rapidly  and  are  already  completed 
early  in  the  second  month. 

The  rotation  of  the  stomach  explains  the  asymmetrical  position  of  the  vagus  nerves 
of  the  adult  organ,  the  left  nerve  supplying  the  ventral  wall  of  the  stomach,  originally  the 
left  wall,  while  the  right  vagus  supplies  the  dorsal  wall,  originally  the  right.  At  the  end 
of  the  seventh  week  the  stomach  has  reached  its  permanent  position,  the  cardia  having 
descended  through  about  ten  segments,  the  pylorus  through  six  or  seven. 


Fig.  94. — Reconstruction  of  a 5 mm.  human  embryo,  showing  the  entodermal  canal  and  its 
derivatives  (His  in  Kollmann).  X 25. 


Gastric  pits  are  indicated  in  embryos  of  seven  weeks,  and  at  14  weeks  the  glands 
begin  to  differentiate.  The  gastric  pits  number  270,000  at  birth  but  increase  by  fission  to 
nearly  seven  million  in  the  adult.  At  seven  w'eeks,  the  circular  muscle  layer  is  indicated  by 
condensed  mesenchyme;  a heavier  ring  forms  the  pyloric  sphincter.  During  the  fourth 
month  the  cardiac  region  shows  a few  longitudinal  muscles  fibers,  which  become  distinct  in 
the  pyloric  region  at  seven  months. 


io6 


THE  UIGESTH'E  SYSTEM 


The  Intestine. — ^In  5 mm.  embryos  (Fig.  93),  the  intestine,  beginning 
at  the  stomach,  consists  of  the  duodenum  (from  which  are  given  off  the 
hepatic  diverticulum  and  dorsal  pancreas),  and  the  cephalic  and  caudal 
limbs  of  the  intestinal  loop,  which  bends  ventrad  and  connects  with  the 
yolk  stalk.  Caudally,  the  intestinal  tube  expands  into  the  cloaca.  It  is 
sup]:)orted  from  the  dorsal  body  wall  by  the  mesentery  (Fig.  94). 

From  5 to  9 mm.,  the  ventral  flexing  of  the  intestinal  loop  becomes  more 
marked  and  the  attachment  of  the  yolk  stalk  to  it  normally  disappears 


Fig. 


Rathke's  pouch 


Hypophysis  {post,  lobe) 
T It  vroid 


Notochord 


Pericardium 


Allantois 


Cloaca!  membrane 

Urogenital  s 


Dorsal  pancreas 
Ventral  pancreas 


Cecum 

Peritoneal  cavity 

Tail  gut  1 Mesonephric  duct 
Rectum 


95. — Median  sagittal  section  of  a 9 mm.  human  embryo,  showing  the  digestive  canal 

(Mall-Prentiss).  X 9. 


Hepatic  duct 
Gall  bladder 
Yolk  stalk 


Trachea 


Esophagus 

Stomach 


Liver 


(Fig.  95).  At  this  stage  there  is  formed  in  the  caudal  limb  of  the  intestinal 
loop  an  enlargement,  due  to  a ventral  bulging  of  the  gut  wall,  that  marks 
the  anlage  of  the  cecum  and  the  boundary  line  between  the  large  and  small 
intestine.  Succeeding  changes  in  the  intestine  consist;  (i)  in  its  torsion 
and  coiling,  due  to  rapid  elongation,  and  (2)  in  the  differentiation  of  its 
several  regions.  As  the  gut  elongates  in  9 to  10  mm.  embryos,  the  intes- 
tinal loop  rotates.  As  a result,  the  originally  caudal  limb  lies  at  the  left 
and  cranial  to  its  cephalic  limb  (Fig.  95). 

f The  small  intestine  soon  lengthens  so  rapidly  that  the  coelom  can  no 
longer  accommodate  it,  and,  at  seven  weeks,  it  protrudes  into  the  umbilical 


THE  UIGESTWE  TUBE 


107 

cord  and  forms  loops  there  (Fig.  96).  This  constitutes  a normal  umbili- 
cal hernia.  Six  primary  loops  occur  and  these  may  be  recognized  in  the 
arrangement  of  the  adult  intestine.  In  embryos  of  ten  weeks,  spatial 
readjustments  have  allowed  the  intestine  to  return  from  the  umbilical 
cord  into  the  abdominal  cavity;  the  coelom  of  the  cord  is  obliterated  soon 
after. 

Vacuoles  appear  in  the  duodenal  wall  of  embryos  six  to  nine  weeks  old  and  epithelial 
septa  completely  block  its  lumen.  The  remainder  of  the  small  intestine  becomes  vacu- 
olated but  not  occluded.  Villi  develop  as  rounded  elevations  of  the  epithelium  at  eight 
weeks.  They  begin  to  form  at  the  cephalic  end  of  the  jejunum,  and,  at  four  months  are 


Fig.  96. — Median  sagittal  section  of  a 17  mm.  human  embryo,  showing  the  digestive  canal 

(Mall-Prentiss).  X 5. 

found  throughout  the  small  intestine.  Intestinal  glands  appear  as  ingrowths  of  the  epithe- 
lium about  the  bases  of  the  villi.  They  develop  first  in  the  duodenum  at  14  weeks.  The 
duodenal  glands  (of  Brunner)  are  said  to  appear  during  the  fourth  month.  In  embryos  of 
six  weeks  the  circular  muscle  layer  of  the  intestine  first  forms,  but  the  longitudinal  layer  is 
not  distinct  until  the  end  of  the  third  month. 

The  large  intestine,  as  seen  in  9 mm.  embryos  (Fig  95),  forms  a tube 
extending  from  the  cecum  to  the  cloaca.  It  does  not  lengthen  so  rapidly 
as  the  small  intestine,  and,  when  the  intestine  is  withdrawn  from  the 
umbilical  cord,  its  cranial,  or  cecal  end  lies  on  the  right  side  and  dorsal 


io8 


THE  DIGESTIVE  SYSTEM 


to  the  small  intestine  (Fig.  97).  It  extends  across  to  the  left  side  as  the 
transverse  colon,  then,  bending  abruptly  caudad  as  the  descending  colon, 
returns  by  its  sigmoid  segment  to  the  median  plane  and  continues  into 
the  rectum.  In  fetuses  from  three  to  six  months  old,  the  lengthening  of 
the  colon  causes  the  cecum  and  cephalic  end  of  the  colon  to  descend  toward 
the  pelvis  (Fig.  97).  The  ascending  colon  is  thus  established  in  the 


Fig.  97. — Later  changes  in  the  intestine  and  dorsal  mesentery  of  the  human  fetus  (Tour- 
neux  in  Heisler).  i.  Stomach;  2,  duodenum;  3,  small  intestine;  4,  colon,  5,  yolk  stalk;  6, 
cecum;  7,  greater  omentum;  8,  mesoduodenum;  9,  mesentery,  10,  mesocolon.  The  arrow  points 
to  the  orifice  of  the  omental  bursa.  The  ventral  mesentery  is  not  shown. 


po.sition  which  it  occupies  in  the  adult.  The  distal  end  of  the  cecal 
anlage  continues  to  elongate,  but  early  lags  in  transverse  development; 
as  a result,  the  vermiform  process  is  distinct  from  the  cecum  at  the  end  of 
the  third  month.  These  structures  make  a sharp  U-shaped  bend  with 
the  colon  at  ten  weeks,  and  this  flexure  gives  rise  to  the  colic  valve. 


Fig. 


98. — Development  of  the  cecum  and  vermiform  process  (adapted  from  Kollman  and  Pater- 
son). A,  Two  months;  B,  three  months;  C,  early  infancy;  D,  five  years. 


The  Rectum. — The  terminal  portion  of  the  intestine  is  derived  by 
the  horizontal  division  of  the  cloaca;  Figs.  95  and  96,  and  140  to  142 
illustrate  the  process  which  is  described  in  full  on  p.  145.  When  the  anal 
membrane  ruptures  at  the  ninth  week,  the  ectodermal  proctodeum  is 
added  to  the  entodermal  rectum. 


THE  LIVER 


lOQ 


The  circular  muscle  layer  of  the  large  intestine  appears  first  at  two  months,  the  longi- 
tudinal layer  at  three  months.  Between  the  third  and  seventh  months  villi  are  present. 

Glandular  secretions  and  desquamated  entodermal  cells,  together  with  swallowed 
amniotic  fluid,  containing  lanugo  hairs  and  vernix  caseosa,  collect  in  the  fetal  intestine. 
This  mass,  yellow  to  brown  in  color,  is  known  as  meconium.  At  birth  the  intestine  and  its 
contents  are  perfectly  sterile,  but  a bacterial  flora  is  promptly  acquired. 

Anomalies. — The  intestine  may  show  atresia.  This  occurs  most  often  in  the  duo- 
denum as  a retention  of  the  embryonic  occlusion.  When  the  anal  membrane  fails  to 
rupture,  an  imperforate  anus  results.  If  the  rectum  does  not  separate  completely  from  the 
cloaca,  a common  urogenital  and  rectal  cavity  remains.  Rarely  there  is  nonrotation  of  the 
intestine  and  the  colon  lies  on  the  left  side.  Two  per  cent  of  all  adults  show  a persistence 
of  the  proximal  end  of  the  yolk  stalk  to  form  a pouch,  Meckel’s  diverticulum  of  the  ileum 
(p.  52).  Congenital  umbilical  hernia  is  due  either  to  the  continuance  of  the  normally 
transitory  embryonic  condition  or  to  a secondary  protrusion  of  the  viscera.  Other  hernias 
are  explained  on  pp.  134  and  163. 


V. 

Fig.  99. — klodel  of  the  liver  anlage  of  a 4 mm.  human  embryo  (Bremer).  X 160.  In., 
Intestine;  Pa.,  pancreas;  V.,  veins  in  contact  with  liver  trabeculse. 


In  embryos  of  2.5  mm.,  the  liver  anlage  is  present  as  a median  ventral 
outgrowth  from  the  entoderm  of  the  fore-gut,  just  cranial  to  the  yolk 
stalk  (Fig.  71).  Its  thick  walls  enclose  a cavity  which  is  continuous  with 
that  of  the  gut.  This  hepatic  diverticitluni  becomes  embedded  at  once 
in  a mass  of  splanchnic  mesoderm,  the  septum  transversum  (Fig.  91). 
Cranially,  the  septum  will  contribute  later  to  the  formation  of  the  dia- 
phragm; caudally,  in  the  region  of  the  liver  anlage,  it  becomes  Glisson’s 
capsule  and  the  ventral  mesentery  (Figs,  no  and  in).  Thus,  from  the 
first,  the  liver  is  in  close  relation  to  the  septum  transversum,  and  later. 


Pa. 


THE  LIVER 


no 


THE  DIGESTIVE  SYSTEM 


when  the  septum  becomes  the  diaphragm,  the  liver  remains  attached  to 
it  (Fig.  113). 

In  embryos  4 to  5 mm.  long,  solid  cords  of  cells  proliferate  from  the 
ventral  and  cranial  portion  of  the  hepatic  diverticulum  (Fig.  91).  These 
cords  anastomose  and  form  a crescentic  mass  with  wings  extending 
u]iward  on  either  side  of  the  gut  (Fig.  93).  This  mass,  a network  of  solid 
trabeula?,  is  the  glandular  portion  of  the  liver,  whereas  the  primitive, 
hollow  diverticulum  differentiates  later  into  the  gall  bladder  and  the 
large  biliary  ducts.  Referring  to  Fig.  183,  it  will  be  seen  that  the  early 
liver  anlage  lies  between  the  vitelline  veins  and  is  in  close  proximity  to 
them  laterally.  The  veins  send  anastomosing  branches  into  the  ventral 
mesentery.  The  trabeculce  of  the  expanding  liver  grow  between  and 
about  these  venous  plexuses,  and  the  plexuses  in  turn  make  their  way 


Fig.  ioo. — The  trabeculae  and  sinusoids  of  the  liver  in  section  (after  Minot).  X 300.  Tr., 
Trabeculae  of  liver  cells;  Si.,  sinusoids. 

between  and  around  the  liver  cords  (Fig.  99).  The  vitelline  veins,  on  their 
way  to  the  heart,  are  thus  surrounded  by  the  liver  and  largely  subdivide 
into  a network  of  vessels,  termed  sinusoids.  The  endothelium  of  the 
sinusoids  is  closely  applied  to  the  cords  of  liver  cells,  which,  in  the  early 
stages,  contain  no  bile  capillaries  (Fig.  100). 

The  glandular  portion  of  the  liver  grows  rapidly,  and,  in  embryos  of 
7 to  8 mm.,  is  connected  with  the  primitive  hepatic  diverticulum  by  a 
single  cord  of  cells  only,  the  hepatic  duct  (Fig.  loi  A).  That  portion  of  the 
hepatic  diverticulum  distal  to  the  hepatic  duct  is  now  differentiated  into 
the  terminal,  solid  gall  bladder  and  its  cystic  duct;  the  proximal  portion 
forms  the  ductus  choledochus.  In  embryos  of  10  mm.  (Fig.  loi  B),  the 
gall  bladder  and  ducts  have  become  longer  and  more  slender  and  the 
hepatic  duct  receives  a right  and  left  branch  from  the  corresponding 
lobes  of  the  liver.  The  gall  bladder  is  without  a lumen  up  to  the  15  mm. 


THE  LIVER 


III 


stage,  but  later  its  cavity  appears,  surrounded  by  a wall  of  high,  columnar 
epithelium. 

The  glandular  portion  of  the  liver  develops  fast  and  is  largest  relative 
to  the  size  of  the  body  at  nine  weeks.  In  certain  regions  the  liver  tissue 


Stomach- 
Hepatic  duct 


Gall  bladder 

Cystic  duct 
Ductus  choledochus 

Ventral  pancreas , 


Gall 


^ bladder 

y V 

/ Cystic  duct'  F | 

1 Dorsal  % 

w pancreas 

Jlcpatic  duct 

Ductus  choledochus 

Ventral  pancreas 


Tail  of 
dorsal 


-Duodenum 


Duct  of  dorsal  ^ 
pancreas 

Head  of  dorsal  pancreas  ' 

Duodenum' 


pancreas 


Fig.  10 1. — Reconstructions  of  the  hepatic  diverticulum  and  pancreatic  anlages  in  human 
embryos.  A,  7.5  mm.  (Thyng).  X 50;  B,  10  mm.  (Prentiss).  X 33. 


undergoes  degeneration,  and  especially  is  this  true  in  the  peripheral 
portion  of  the  left  lobe.  In  general,  the  external  lobes  of  the  liver  are 
moulded  under  the  influence  of  the  fetal  vitelline  and  umbilical  trunks. 
The  development  of  the  ligaments  of  the  liver  is  described  on  p.  126. 


Fig.  102. — Diagrams  illustrating  the  formation  of  liver  lobules  (Mall),  a,  Hepatic  side; 
d,  portal  side;  b and  c,  successive  stages  of  the  hepatic  vein;  e and  f,  successive  stages  of  the 
portal  vein. 


During  the  development  of  the  liver  the  endothelial  cells  of  the  sinusoids  become 
stellate  in  outline,  and  thus  form  an  incomplete  layer.  From  the  second  month  of  fetal 
life  to  some  time  after  birth,  blood  cells  are  actively  differentiating  between  the  hepatic 
cells  and  the  endothelium  of  the  sinusoids.  At  eight  weeks  hollow  interlobular  ducts  appear. 


112 


THE  DIGESTR  E SYSTEM 


Spreading  inward  from  the  hepatic  duct  along  the  larger  branches  of  the  portal  vein.  In 
fetuses  of  ten  weeks  bile  capillaries  with  cuticular  borders  are  present,  most  numerous 
near  the  interlobular  ducts  with  which  some  of  them  connect.  At  birth,  or  shortly  after, 
the  number  of  liver  cells  surrounding  a bile  capillary  is  reduced  to  two,  three,  or  four. 
Secretion  of  the  bile  commences  at  about  the  end  of  the  third  fetal  month. 

The  lobules,  or  vascular  units  of  the  liver,  are  formed,  according  to  Mall,  by  the  pecu- 
liar and  regular  manner  in  which  the  veins  of  the  liver  branch.  The  primary  branches 
of  the  portal  vein  extend  along  the  periphery  of  each  primitive  lobule,  parallel  to  similar 
branches  of  the  hepatic  veins  that  drain  the  blood  from  the  center  of  the  lobule  (Fig.  102). 
As  development  proceeds,  each  primary  branch  becomes  a stem,  giving  off  on  either  side 
secondary  branches  which  bear  the  same  relation  to  each  other  and  to  new  lobules  as  did 
the  primary  branches  to  the  first  lobule.  This  process  is  repeated  during  fetal  and  early 
postnatal  life  until  thousands  of  liver  lobules  are  developed. 

Until  the  20  mm.  stage,  the  portal  vein  alone  supplies  the  liver.  The  hepatic  artery, 
from  the  cadiac  axis,  comes  into  relation  first  with  the  hepatic  duct  and  gall  bladder. 
Later,  it  grows  into  the  connective  tissue  about  the  larger  bile  ducts  and  the  branches  of 
the  portal  vein,  and  also  supplies  the  capsule  of  the  liver. 

Anomalies. — A common  anomaly  of  the  liver  consists  in  its  subdivision  into  multiple 
lobes.  Absence  or  duplication  of  the  gall  bladder  and  of  the  ducts  may  occur.  In  some 
animals  (horse;  elephant)  the  gall  bladder  is  absent  normally. 


THE  PANCREAS 

Two  pancreatic  anlages  are  developed  almost  simultaneously  in 
embryos  of  3 to  4 mm.  The  dorsal  pancreas  arises  as  a hollow  outpocket- 
ing  of  the  dorsal  duodenal  wall,  just  cranial  to  the  hepatic  diverticulum 


' Dorsal  pancreatic  duct 
Dorsal  pancreas 


Ventral  pancreas 
Ventral  pancreatic  duct 

Bile  duct 

Fig.  103. — Stages  in  the  development  of  the  human  pancreas  (Kollman). 


4 , 8 mm. ; B,  20  mm. 


(Figs.  93  and  94).  At  7.5  mm.,  it  is  separated  from  the  duodenum  by  a 
slight  constriction  and  extends  into  the  dorsal  mesentery  (Fig.  10 1 A). 
The  ventral  pancreas  develops  in  the  inferior  angle  between  the  hepatic 
diverticulum  and  the  gut,  and  its  wall  is  at  first  continuous  with  both. 
With  the  elongation  of  the  ductus  choledochus,  its  origin  is  transferred  to 
this  portion  of  the  diverticulum. 

Of  the  two  pancreatic  anlages,  the  dorsal  grows  more  rapidly,  and,  in 
10  mm.  embryos,  forms  an  elongated  structure  with  a central  duct  and 


i 


THE  PANCREAS 


II3 

irregular  nodules  upon  its  surface  (Fig.  loi  B).  The  ventral  pancreas 
is  smaller  and  develops  a short,  slender  duct  that  opens  into  the  ductus 
choledochus.  When  the  stomach  and  duodenum  rotate,  the  pancreatic 
ducts  shift  their  positions  as  well.  At  the  same  time,  growth  and  bending 
of  the  bile  duct  to  the  right  bring  the  ventral  pancreas  into  close  proximity 
with  the  dorsal  pancreas  (Figs.  loi  and  103). 

In  embryos  of  20  mm.,  the  tubules  of  the  dorsal  and  ventral  pancreatic 
anlages  interlock  (Fig.  103  B).  Eventually,  anastomosis  takes  place 
between  the  two  ducts,  and  the  duct  of  the  ventral  pancreas,  plus  the  distal 
segment  of  the  dorsal  duct,  persists  as  the  functional  pancreatic  duct 
(of  Wirsung)  of  the  adult.  The  proximal  portion  of  the  dorsal  pancreatic 
duct  forms  the  accessory  duct  (of  Santorini),  which  remains  pervious, 
but  becomes  a tributary  of  the  chief  pancreatic  duct.  The  ventral 
pancreas  forms  part  of  the  head  and  uncinate  process  of  the  adult  gland. 
The  dorsal  pancreas  participates  in  forming  the  head  and  uncinate  process, 
and  comprises  the  whole  of  the  body  and  tail. 

In  10  mm.  embryos  the  portal  vein  separates  the  two  pancreatic  anlages,  and  later 
they  partially  surround  the  vein.  The  alveoli  of  the  gland  are  derived  from  the  ducts  as 
darkly  staining  cellular  buds  in  fetuses  of  ten  weeks.  The  islands,  characteristic  of  the 
pancreas,  also  bud  from  the  ducts  (and  alevoli,  Mironescu,  1910)  and  appear  first  in  the  tail 
a week  later.  Owing  to  the  shift  in  the  position  of  the  stomach  and  duodenum  during 
development,  the  pancreas  takes  up  a transverse  position. 

Anomalies. — The  ventral  pancreas  may  arise  directly  from  the  intestinal  wall,  and 
paired  ventral  anlages  also  occur.  Accessory  pancreases  are  not  uncommon.  Both  the 
dorsal  and  ventral  ducts  persist  in  the  horse  and  dog;  in  the  sheep  and  man,  the  ventral 
duct  normally  becomes  of  chief  importance;  in  the  pig  and  ox,  the  dorsal  duct. 


8 


CHAPTER  VI 


THE  RESPIRATORY  SYSTEM 

The  development  of  the  nose  and  pharynx  are  described  elsewhere. 
Accordingly,  the  present  account  will  deal  exclusively  with  the  origin  of 
the  other  respiratory  organs.  In  embryos  of  23  segments,  the  anlage  of 
this  apparatus  appears  as  a laryngo-tracheal  groove  in  the  floor  of  the 
entodermal  tube,  just  caudal  to  the  pharyngeal  pouches.  This  groove  pro- 
duces an  external  ridge  on  the  ventral  wall  of  the  tube  which  promptly 
becomes  larger  and  rounded  at  its  caudal  end  (Fig.  104).  The  groove  and 


Fig.  104. — Stages  in  the  early  development  of  the  trachea  and  lungs  of  human  embryos 
(adapted  by  Prentiss).  X about  50.  A,  2.5  mm.;  B,  4 mm.;  C,  stage  B in  side  view;  D, 
5 mm. ; E,  7 mm. 

the  ridge  are  the  anlages  of  the  larynx  and  trachea.  The  rounded  end  of 
the  ridge  is  the  unpaired  anlage  of  the  lungs;  in  embryos  of  4 to  5 mm. 
it  becomes  bilobed. 

Externally,  two  lateral  longitudinal  grooves  mark  off  the  dorsal 
esophagus  from  the  ventral  respiratory  anlages.  A fusion  of  the  lateral 
furrows,  progressing  cephalad,  constricts  first  the  lung  anlages  and  then 
the  trachea  from  the  esophagus.  At  the  same  time  the  laryngeal  portion 
of  the  groove  and  ridge  advances  cranially  until  it  lies  between  the  fourth 

114 


THE  LARYNX 


II5 


branchial  arches  (Fig.  87).  At  5 mm.,  the  respiratory  apparatus  consists 
of  the  laryngeal  groove  and  ridge,  the  tubular  trachea,  and  the  two  lung 
buds  (Fig.  104  D). 

The  Larynx. — In  embryos  of  5 to  6 mm.,  the  oral  end  of  the  laryngeal 
groove  is  bounded  on  either  side  by  two  rounded  prominences,  the  aryte- 
noid swellings,  which  are  continuous  orally  with  a transverse  ridge  to 


Fig.  105. — The  larynx  of  a 16  mm.  human  embryo  (Kallius).  X 15. 


form  the  fiirciila  of  His  (Fig.  85).  The  transverse  ridge  becomes  the 
epiglottis,  derived  from  the  third  and  fourth  branchial  arches  (p.  g6). 
In  embryos  of  15  mm.,  the  arytenoid  swellings  are  bent  near  the  middle; 
their  caudal  portions  lie  parallel,  while  their  cephalic  segments  diverge 


nearly  at  right  angles  (Fig.  105).  The  glottis,  opening  into  the  larynx, 
thus  becomes  T-shaped  and  ends  blindly,  as  the  laryngeal  epithelium  has 
fused.  In  fetuses  of  ten  weeks  this  fusion  is  dissolved,  the  arytenoid 
swellings  are  withdrawn  from  contact  with  the  epiglottis,  and  the  entrance 
to  the  larynx  becomes  oval  in  form  (Fig.  106).  At  eight  weeks  the  ven- 


ii6 


THE  RESPIRATORY  SYSTEM 


tndes  of  the  larynx  appear,  and,  during  the  tenth  week,  their  margins 
indicate  the  position  of  the  vocal  cords.  The  elastic  and  muscle  fibers 
of  the  cords  are  developed  by  the  fifth  month. 

At  the  end  of  the  sixth  week,  the  cartilaginous  skeleton  of  the  larynx  is  indicated  by 
surrounding  condensations  of  mesenchyme,  derived  from  the  fourth  and  fifth  pairs  of 
branchial  arches  (p.  221).  The  cartilage  of  the  epiglottis  appears  relatively  late. 
The  thyroid  cartilage  is  formed  from  the  fusion  of  two  lateral  plates,  each  of  which  has  two 
centers  of  chondrification.  The  anlages  of  the  cricoid  and  arytenoid  cartilages  are  originally 
continuous;  later,  separate  cartilage  centers  develop  for  the  arytenoids.  The  cricoid  is  at 
first  incomplete  dorsad,  but  eventually  forms  a complete  ring;  it  therefore  may  be  regarded 
as  a modified  tracheal  ring.  The  corniculatc  cartilages  represent  separated  portions  of  the 
arytenoids.  The  cuneiform  cartilages  are  derived  from  the  cartilage  of  the  epiglottis. 
The  laryngeal  muscles  originate  in  the  fourth  and  fifth  branchial  arches  and  are  conse- 
quently innervated  by  the  vagus  nerve  which  supplies  those  arches. 

The  Trachea.  —This  tube  gradually  elongates  during  development, 
and  its  columnar  epithelium  becomes  ciliated.  Muscle  fibers  and  the 
anlages  of  the  cartilaginous  rings  appear  in  the  condensed  mesenchyme 
at  the  end  of  the  seventh  week.  The  glands  develop  as  ingrowths  of 
the  epithelium  during  the  last  five  months  of  fetal  life. 

The  Lungs. — Soon  after  the  lung  anlages,  or  stem  buds,  are  formed 
(in  5 mm.  embryos),  the  right  bronchial  bud  becomes  larger  and  is  directed 


Fig.  107. — Ventral  and  dorsal  views  of  the  lungs  from  a human  embryo  of  about  9 mm. 
(after  Merkel).  Ap.,  Apical  bronchus;  Di,  D2,  etc.,  dorsal,  V i.  V2,  ventral  bronchi; 
Jc.,  infracardiac  bronchus. 

straighter  caudad  (Fig.  104).  At  7 mm.  the  stem  bronchi  give  rise  to  two 
bronchial  buds  on  the  right  side,  to  one  on  the  left.  The  smaller  bronchial 
bud  on  the  right  side  is  the  apical  {eparterial)  hud.  The  right  and  left 
chief  buds,  known  as  ventral  bronchi,  soon  bifurcate.  There  are  thus  formed 
three  bronchial  rami  on  the  right  side  and  two  on  the  left;  these  cor- 
respond to  the  primitive  lobes  of  the  lungs  (Figs.  88  and  107).  On  the 
left  side,  an  apical  bud  is  interpreted  as  being  derived  from  the  first  ven- 
tral bronchus  (Fig.  107).  It  remains  small  so  that  there  is  no  separate 
lobe  corresponding  to  the  upper  lobe  of  the  right  lung.  The  absence  of 


THE  LUNGS 


II7 


this  upper  left  lobe  may  be  an  adaptation  to  permit  the  normal  caudal 
regression  of  the  aortic  arch  (p.  190). 

The  bronchial  anlages  continue  to  branch  in  srich  a way  that  the  stem 
bud  is  retained  as  the  main  bronchial  stem  (Fig.  107).  That  is,  the 
branching  is  monopodial,  not  dichotomous,  lateral  buds  being  given  off 
from  the  stem  bud  proximal  to  its  growing  tip.  Only  in  the  later  stages 
of  development  has  dichotomous  branching  of  the  bronchi  and  the  forma- 
tion of  two  equal  buds  been  described.  Such  buds,  formed  dichotomously, 
do  not  remain  of  equal  size  (Flint).  The  inclination  of  the  heart  to  the 
left  suppresses  one  of  the  larger  ventral  bronchial  rami  on  that  side,  but 
at  the  same  time  it  affords  opportunity  for  an  excessive  development  of 


Fig.  108. — Transverse  section  through  the  lungs  and  pleural  cavities  of  a 10  mm  human 

embryo  (Prentiss).  X 23. 


the  corresponding  right  ramus  which  then  projects  into  the  space  between 
the  heart  and  diaphragm  as  the  infrasardiac  bronchus  (Fig.  107,  Jc). 

The  entodermal  anlages  of  the  lungs  and  trachea  are  developed  in  a 
median  mass  of  mesenchyme,  dorsal  and  cranial  to  the  peritoneal  cavity. 
This  tissue  forms  a broad  mesentery,  termed  the  mediastinum  (Fig.  108). 
The  right  and  left  stem  buds  of  the  lungs  grow  out  laterad,  carrying  with 
them  folds  of  the  mesoderm.  The  branching  of  the  bronchial  buds 
takes  place  within  this  tissue  which  is  covered  by  the  mesothelial  lining 
of  the  future  pleural  cavity.  The  terminal  branches  of  the  bronchi  are 
lined  with  entodermal  cells;  these  flatten  out  and  form  the  respiratory 
epithelium  of  the  adult  lungs.  The  surrounding  mesenchyme  differ- 


THE  RESPIRATORY  SYSTEM 


1 18 

entiates  into  the  muscle,  connective  tissue,  and  cartilage  plates  of  the 
lung,  tracheal,  and  bronchial  walls.  Into  it  grow  blood  vessels  and  nerve 
fibers.  When  the  pleural  cavities  are  separated  from  the  pericardial 
and  ])eritoneal  cavities,  the  mesothelium  covering  the  lungs,  with  the 
connective  tissue  underl3ung  it,  becomes  the  visceral  pleura.  The  cor- 
responding layers  lining  the  thoracic  wall  form  the  parietal  pleura.  These 
layers  are  derived  respectively  from  the  visceral  (splanchnic)  and  parietal 
(somatic)  mesoderm  of  the  embryo. 

In  II  mm.  embryos,  the  two  pulmonary  arteries,  from  the  sixth  pair  of 
aortic  arches,  course  first  lateral  then  dorsal  to  the  stem  bronchi  (Fig. 
109).  d he  right  pulmonary  artery  passes  ventral  to  the  apical  (epar- 

terial)  bronchus  of  the  right  lung. 
The  single  pulmonary  vein  receives 
two  branches  from  each  lung:  a 
larger  vein  from  each  lower  lobe,  a 
smaller  vein  from  each  upper  lobe, 
including  the  middle  lobe  of  the 
right  side.  These  four  pulmonary 
branches  course  ventrad  and  drain 
into  the  pulmonary  trunk.  When 
this  common  stem  is  taken  up  into 
the  wall  of  the  left  atrium,  the  four 
pulmonary  veins  open  directly  in 
to  the  latter. 

According  to  Kolliker,  the  air  cells,  or  alveoli,  of  the  lungs  begin  to  form  in  the  sixth 
month  and  their  developnent  is  completed  during  pregnancy.  Elastic  tissue  appears 
during  the  fourth  month  in  the  largest  bronchi.  The  abundant  connective  tissue  found 
between  the  bronchial  branches  in  early  fetal  life  becomes  reduced  in  its  relative  amount  as 
the  alveoli  of  the  lungs  are  developed. 

Until  birth  the  lungs  are  relatively  small,  compact,  and  possess  sharp  margins.  They 
lie  in  the  dorsal  portion  of  the  pleural  cavities.  After  birth  the  lungs  normally  fill  with  air, 
expanding  and  completely  filling  the  pleural  cavities.  Their  margins  are  then  rounded 
and  the  compact,  fetal  lung  tissue,  which  resembles  that  of  a gland  in  strrrcture,  becomes 
light  and  spongy,  owing  to  the  enormous  increase  in  the  size  of  the  alveoli  and  blood  vessels. 
Because  of  the  greater  amount  of  blood  admitted  to  the  lungs  after  birth,  their  weight  is 
suddenly  increased. 

Anomalies. — Variations  occur  in  the  size  and  number  of  lobes  of  the  lungs;  rarely 
there  is  a third  lobe  on  the  left  side.  The  most  common  anomaly  involving  both  esopha- 
gus and  trachea  is  described  on  p.  104. 

A striking  malformation  of  the  viscera  in  general  is  situs  visccrum  inversus,  in  which  the 
various  organs  are  transposed  in  position,  right  for  left  and  left  for  right,  as  in  a mirror 
image.  This  reversal  may  effect  all  the  internal  organs,  or  an  independent  transposition 
of  the  thoracic  or  abdominal  viscera  alone  may  occur.  The  early  influence  of  the  larger 
left  great  venous  trunks  is  thought  to  be  chiefly  responsible  for  the  usual  positions  and 
asymmetrical  relations  of  the  viscera. 


Fig.  109. — Ventral  view  of  the  lungs  of  a 10.5 
mm.  embryo,  showing  the  pulmonary  arteries 
and  veins  (His  in  McMurrich).  X 27.  Ep., 
Apical  bronchus;  I,  II,  primary  bronchi. 


MESODERMAL  DERIVATIVES 


CHAPTER  VII 

THE  MESENTERIES  AND  CCELOM 

I.  THE  MESENTERIES 


The  Primitive  Mesentery. — -The  gut  arises  when  the  entoderm  is 
folded  into  a tube  (Fig.  165).  At  the  same  time,  the  lateral  expanse  of 
superposed  splanchnic  mesoderm  swings  inward  from  each  side  toward 
the  midplane  and  forms  a double-layered  sheet,  extending  from  the  roof 


Esophagus  Spinal  cord 


Mesentery 


Mesocolon 


Dorsal  mesocar- 
diiun 

A 

Septum  transversum 
Stomach 

Ventral  mesentery 
(lesser  omentum) 
Dorsal  mesogastriiim 
Dorsal  pancreas 


Pericardial  cavity 
Ventricle  of  heart 

Ventral  mesocardium 
Liver 

Ventral  mesentery 
(falciform  ligament) 


Mesorectum 


Fig.  1 10. — Diagram  showing  the  primitive  mesenteries  of  an  early  human  embryo  in 
median  sagittal  section  (Prentiss).  The  broken  lines  (A,  B,  and  C)  indicate  the  level  of  sections 
A , B,  and  C,  in  Fig.  1 1 1 . 


of  the  coelom  to  the  midventral  body  wall  and  containing  the  gut  between 
its  layers  (Figs,  no  and  in).  This  membrane  is  the  primitive  mesentery. 
The  covering  layers  of  the  gut  (and  other  viscera),  mesenteries,  and  body 
wall  are  continuous  with  each  other  and  consist  of  a mesothelium,  over- 

119 


120 


THE  MESENTERIES  AND  CCELOM 


lying  connectiA^e  tissue  (Fig.  iii).  The  parietal  lining  is  derived  from 
the  somatic  layer  of  mesoderm  and  the  visceral  covering  from  the  splanch- 
nic layer  (Fig.  165). 


Neural  fuhe 

Notochord 
Dorsal 
ynesentery 
Fore-f!,ut 
Dorsal 

cardium 

Heart 


Ventral 


A 


Newal  tube 


Fi(..  I 1 1. — Diagrammatic  transverse  sections  of  an  early  human  embryo  (Prentiss).  A, 
Through  the  heart  and  pericardial  cavities;  B,  through  the  fore-gut  and  liver;  C,  through  the 
intestine  and  peritoneal  cavity.  (Compare  guide  lines  A,  B,  C,  Fig.  iio.) 


Differentiation  of  the  Dorsal  Mesentery. — At  first,  the  gut  is  broadly 
attached  dorsad  and  its  roof  lies  directly  beneath  the  notochord  and 


Jiiglit  umhiliral  viin  Left  umbilical  vein 


Ectoderm  of  body  wall 

Left  lobe  of  liver 
Ventral  mesentery 
Duodenum 

Dorsal  mesentery 
Left  posterior  cardinal  vein 
Notochord 


Ventral  mesentery 


Right  lobe  of  liver 
Omental  bursa 
Caval 

Dorsal  aoric 

Neural  tube 


F''ig.  1 1 2. — Diagrammatic  model  cf  an  8 mm.  human  embryo,  showing  the  primitive 
omental  bursa  (Prentiss).  The  model  is  sectioned  transversely,  caudal  to  the  liver,  so  the 
observer  looks  cephalad  at  the  caudal  surfaces  of  the  liver  and  sectioned  body. 


descending  aortee  (Fig.  165).  Presently  this  region  of  attachment 
becomes  relatively  narrow,  and  the  gut  is  then  suspended  throughout  most 
its  length  by  a definite  dorsal  mesentery  which  extends  like  a curtain  in 


THE  MESENTERIES 


I2I 


the  midplane.  The  esophagus  lies  in  the  mediastinum  and  has  no  typical 
mesentery  in  the  adult  (Fig.  124).  On  the  contrary,  that  portion  of  the 
digestive  canal  which  passes  through  the  peritoneal  cavity  is  contained 
in  an  originally  continuous  dorsal  mesentery.  Later,  distinctive  names 
are  given  to  its  several  regions  (Fig.  no):  thus,  there  is  the  dorsal  meso- 
gastrium  (or  greater  omentum),  of  the  stomach,  the  mesoduodeniim,  the 
mesentery  proper  of  the  small  intestine,  the  mesocolon,  and  the  mesorecinm. 

The  Omental  Bursa. — The  history  of  the  mesogastrium  is  chiefly 
concerned  with  the  development  of  a huge  sacculation  known  as  the 
omental  bursa,  or  lesser  peritoneal  sac.  According  to  Broman,  its  first 
indication  in  a 3 mm.  embryo  is  a peritoneal  pocket  which  extends  cra- 
nially  into  the  dorsal  mesentery,  to  the  right  of  the  esophagus.  A similar 
pocket,  present  on  the  left  side,  has  disappeared  in  4 mm.  embryos.  Lateral 
to  the  opening  of  the  primitive  bursa,  a lip-like  fold  of  the  mesentery  is 
continued  caudally  along  the  dorsal  body  wall  into  the  mesonephric  fold 
as  the  caval  mesentery,  in  which  the  inferior  vena  cava  develops  later 
(Fig.  1 12).  Furthermore,  it  will  be  remembered  that  the  liver  grows  out 
into  the  ventral  mesentery  from  the  fore-gut,  and,  expanding  laterally 
and  ventrally,  takes  the  form  of  a cresent.  Its  right  lobe  comes  into 
relation  with  the  caval  mesentery,  and,  growing  rapidly  caudad,  forms 
with  this  fold  a partition  between  the  lesser  sac  and  the  peritoneal  cavity. 
Thus,  the  cavity  of  the  omental  bursa  is  extended  caudally  from  a point 
opposite  the  bifurcation  of  the  lungs  to  the  level  of  the  pyloric  end  of  the 
stomach.  In  5 to  10  mm.  embryos,  it  is  crescent-shaped  in  cross  section 
(cf.  Fig.  Ill)  and  is  bounded  mesially  by  the  greater  omentum  (dorsal 
mesentery)  and  the  right  wall  of  the  stomach,  laterally  by  the  liver  and 
caval  mesentery,  and  ventrally  by  the  lesser  omentum  (ventral  mesentery) 
(Fig.  1 14).  It  communicates  to  the  right  with  the  peritoneal  cavity 
through  an  opening  between  the  liver  ventrally  and  the  caval  mesentery 
dorsally  (Figs.  114  and  116).  This  aperture  is  the  epiploic  foramen  (of 
Winslow).  When  the  dorsal  wall  of  the  stomach  rotates  to  the  left,  the 
greater  omentum  is  carried  with  it  to  the  left  of  its  dorsal  attachment. 
The  omental  tissue  grows  actively  to  this  side  and  caudally,  and  gives  the 
omentum  an  appearance  of  being  folded  on  itself  between  the  stomach 
and  the  dorsal  body  wall  (Fig.  113).  The  cavity  of  the  omental  bursa  is 
carried  out  between  the  folds  of  the  greater  omentum  as  the  inferior  recess 
(Figs.  97  and  116). 

From  the  cranial  end  of  the  sac  there  is  constricted  off  a small  closed  cavity  which  is 
frequently  persistent  in  the  adult.  This  is  the  in  fracardiac  bursa  and  may  be  regarded  as  a 
third  pleural  cavity.  It  lies  at  the  right  of  the  esophagus  in  the  mediastinum. 

When  the  stomach  changes  its  position  and  form  so  that  its  mid- 
ventral  line  becomes  the  lesser  curvature  and  lies  to  the  right,  the  position 


122 


THE  MESENTERIES  AND  CCELOM 


of  the  lesser  omentum  is  also  shifted.  From  its  xirimitive  location  in  a 
median  sagittal  xdane,  with  its  free  edge  directed  caudally,  the  lesser 
omentum  is  rotated  through  90°  until  it  lies  in  a coronal  plane  with  its 
free  margin  facing  to  the  right.  The  epiploic  foramen  then  forms  a slit- 
like o|Dening  leading  from  the  peritoneal  cavity  into  the  vestibule  of 
the  omental  bursa  (Fig.  114).  The  foramen  is  bounded  ventrally  by  the 
edge  of  the  lesser  omentum,  dorsally  by  the  inferior  vena  cava,  cranially 


Body  wall 


In  ferior  vena  cava 

Sup.  recess  of  lesser 
peritoneal  sac 

Plciiro-periloncal 
membrane 

Inferior  vena  cava 

^Caval  mesentery 


Mesonephric  fold 
Genital  fold 


Falciform  ligament 


Coronary  attachment  of 
liver  to  diaphragm 


reater  omentum 
Spleen 
Stomach 


Plenro-peritoneal 

foramen 


Plenro-peritoneal 

membrane 


Lesser  omentum 


Omental  bursa 


Aorta 


Fig.  1 13. — Diagrammatic  model  of  a 14  mm.  human  embryo  (adapted  by  Prentiss).  The 
figure  shows  the  caudal  surface  of  a section  through  the  stomach  and  spleen,  a ventral  view  of 
the  stomach,  the  liver  having  been  cut  away  to  leave  the  sectioned  edges  of  the  omental  bursa 
and  caval  mesentery,  and  the  caudal  surface  of  the  septum  transversum  and  pleuro-peritoneal 
membrane.  Upon  the  surface  of  the  septum  is  indicated  the  attachment  of  the  liver. 


by  the  caudate  x^rocess  of  the  liver,  and  caudally  by  the  wall  of  the 
doudenum. 

During  fetal  life  the  greater  omentum  grows  rapidly  to  the  left  and 
caudad,  in  the  form  of  a sac,  flattened  dorso-ventrally.  It  overlies  the 
intestines  ventrally  and  contains  the  inferior  recess  of  the  omental  bursa 
(Fig.  1 1 5).  In  the  fourth  month,  the  dorsal  wall  of  the  sac  usually  fuses 
with  the  transverse  colon  and  mesocolon  where  it  overlies  them  (Fig. 


THE  MESENTERIES 


123 


Fig.  1 14. — Transverse  section  through  a 10  mm.  human  embryo  at  the  level  of  the  stomach  and 

epiploic  foramen  (Prentiss).  X 33. 


Fig.  1 1 5. — Diagrams  showing  the  developmental  relations  of  the  greater  omentum  (Hert- 
wig).  A,  Illustrates  the  beginning  of  the  greater  omentum  and  its  independence  of  the  trans- 
verse mesocolon:  in  B the  two  come  into  contact;  in  C they  have  fused.  A,  Stomach;  B, 
transverse  colon;  C,  small  intestine;  D,  duodenum;  E,  pancreas;  F,  greater  omentum;  G,  greater 
sac;  H,  omental  bursa. 


124 


THE  MESENTERIES  AND  C(EL()M 


1 15  B).  The  transverse  mesocolon  of  the  adult  is  conseciuently  a double 
structure  and  the  omental  connection  between  stomach  and  colon  becomes 
the  gastro-coUc  ligament.  Caudal  to  this  attachment,  the  walls  of  the 
omental  bursa  commonly  unite  and  obliterate  its  cavity.  The  inferior 
recess  of  the  omental  bursa  thus  may  be  limited  in  the  adult  chiefly  to  a 
space  between  the  stomach  and  the  dorsal  fold  of  the  great  omentum, 
which  latter  is  largely  fused  to  the  peritoneum  of  the  dorsal  body  wall. 
The  s[)leen  develops  in  the  cranial  portion  of  the  great  omentum;  that 
stretch  of  the  omentum  extending  between  the  stomach  and  spleen  is 
known  as  the  gastro-splenic  ligament  (Fig.  113),  while  its  continuation 
beyond  the  spleen  is  the  spleno-renal  ligament. 

Other  L hanges  in  the  Dorsal  Mesentery. — As  long  as  the  gut  remains  a 
straight  tube,  the  dorsal  mesentery  is  a simple  sheet  whose  two  attached 


Lesser  omentum 


•Stomach 


Fig.  1 16. — Diagrams  showing  the  later  history  of  the  dorsal  mesentery  in  ventral  view 
(Tourneux-Prentiss).  *,  Cut  edge  of  greater  omentum;  a,  h,  area  of  ascending  and  descending 
mesocolon  fused  to  dorsal  body  wall.  The  arrow  emerges  from  the  omental  bursa. 

edges  are  equal  in  length.  But  when  the  intestine  starts  to  elongate 
faster  than  the  body  wall,  and  forms  first  a loop  and  then  coils,  the  intes- 
tinal attachment  of  the  mesentery  grows  correspondingly  and  is  carried 
out  into  the  umbilical  cord  between  the  intestinal  limbs.  Even  before 
this  herniation  of  the  intestine  occurs,  its  limbs  are  so  shifted  that  the 
cecal  end  of  the  large  intestine  comes  to  lie  cranially  and  to  the  left,  and 
the  small  intestine  caudally  and  to  the  right;  in  this  position  the  future 
duodenum  and  colon  cross  in  close  proximity  to  each  other  (Fig.  95). 
On  the  return  of  the  intestinal  loop  into  the  abdomen,  the  cecal  end  of 


THE  MESENTERIES 


125 


the  colon  is  carried  over  to  the  right,  and  the  transverse  colon  crosses 
the  duodenum  ventrally  and  cranially  (Fig.  116  A).  The  primary  loops 
of  the  small  intestine  lie  caudal  and  to  the  left  of  the  ascending  colon. 
There  has  thus  been  a torsion  of  the  mesentery  about  the  origin  of  the 
superior  mesenteric  artery  as  an  axis,  which  is  accentuated  as  the  limb  of 
the  ascending  colon  elongates  toward  the  pelvis  (Fig.  116  B).  From  this 
focal  point,  the  mesentery  of  the  small  intestine  and  colon  spreads  out 
like  a fan  or  funnel. 

Previous  to  the  middle  of  the  fourth  month,  the  gut  is  freely  movable 
within  the  scope  of  its  restraining  mesentery,  but  soon  secondary  fusions 
occur  which  attach  certain  segments.  The  ascending  and  descending 
colon  are  applied  against  the  body  wall  on  the  right  and  left  side  respec- 
tively. The  flat  surfaces  of  their  mesenteries  fuse  with  the  adjacent  dorsal 
peritoneum,  and  these  two  limbs  of  the  colon  become  permanently  an- 
chored (Fig.  1 16).  vSince  the  transverse  colon  passes  ventral  to  the  duo- 
denum (Fig.  1 16),  its  mesentery  remains  distinct;  but  in  the  region  of 
crossing,  the  base  of  the  mesocolon  fuses  with  the  surface  of  the  duodenum 
and  pancreas.  In  accordance  with  its  final  position,  this  mesentery  is 
now  known  as  the  transverse  mesocolon.  The  line  of  attachment  of  the 
mesocolon  presses  the  duodenum  firmly  against  the  dorsal  body  wall  and 
obliterates  its  mesentery,  thereby  fixing  this  portion  of  the  small  intestine. 
The  pancreas,  which  primarily  is  an  outgrowth  of  the  duodenum  into  the 
mesoduodenum,  necessarily  assumes  also  a retroperitoneal  position  behind 
the  root  of  the  transverse  mesocolon.  The  mutual  union  of  the  lamellae 
of  the  greater  omentum  and  its  fusion  to  the  transverse  colon  and  the  dorsal 
body  wall  have  been  mentioned.  The  mesentery  proper  of  the  small 
intestine  is  thrown  into  numerous  folds,  corresponding  to  the  loops  of  the 
intestine,  but  normally  does  not  exhibit  secondary  attachments;  the 
sigmoid  mesocolon  likewise  remains  free. 

Differentiation  of  the  Ventral  Mesentery. — The  same  splanchnic 
mesodermal  layers  that  invest  the  entoderm  and  form  the  dorsal  mesen- 
tery, also  combine  beneath  the  gut  as  the  ventral  mesentery  (Figs,  no 
and  III).  The  ventral  mesentery  is  associated  intimately  with  the 
development  of  two  important  organs.  One  is  the  heart,  which  becomes 
a single  tube  by  the  union  of  paired  anlages  lying  one  in  each  lateral  fold 
of  splanchnic  mesoderm  (Fig.  165).  Hence  the  heart  is  supported  by 
the  ventral  mesentery  both  above  and  below  (Figs,  no  and  in  A).  The 
other  organ,  the  liver,  grows  downward  into  the  ventral  mesogastrium, 
splitting  apart  its  component  lamellae  and  then  having  similar  mesen- 
terial relations  as  the  primitive  heart  (Figs,  no  and  in  B).  Caudal  to 
the  yolk  sac  the  ventral  mesentery  does  not  persist,  even  in  young  embryos 
(Figs,  no  and  in  C). 


126 


THE  MESENTERIES  AND  COELOM 


Most  ce])halad,  the  heart  is  suspended  in  the  ventral  mesentery  which 
is  there  designated  the  dorsal  and  ventral  mesocardium  (Figs,  -no  and 
III  *4).  The  latter  is  transitory  and  the  dorsal  mesocardium  also  dis- 
apjiears  somewhat  later,  leaving  the  heart  unsupported  in  the  pericardial 
cavity  (Fig.  i66). 

Ligaments  of  the  Liver. — From  the  first,  the  liver  is  enclosed  by  the 
lamelte  of  the  ventral  mesogastrium,  which,  as  the  liver  increases  in  size, 
give  rise  to  its  capsule  and  ligaments  (Figs,  no  and  in  B).  Wherever 
the  liver  is  unattached,  the  enveloping  mesodermal  layers  form  the  capsule 
(of  Glisson),  a fibrous  layer  covered  by  mesothelium,  continuous  with 
that  of  the  jjeritoneum  (Fig.  in  B).  Along  its  mid-dorsal  and  mid- 
ventral  line,  the  liver  remains  connected  to  the  ventral  mesentery.  That 
portion  of  the  mesentery  between  the  liver,  stomach,  and  duodenum  is 
the  lesser  omentum  (Fig.  113).  This  in  the  adult  is  differentiated  into 
the  hepato-gastric  and  he  pato-diiodcnal  ligaments.  The  mesogastrial 
attachment  of  the  liver  to  the  ventral  body  wall  extends  caudally  from 
diaphragm  to  umbilicus  and  constitutes  the  falciform  ligament. 

In  its  early  development,  the  liver  abuts  upon  the  primitive  dia- 
phragm, and,  in  4 to  5 mm.  embryos,  is  attached  to  it  along  its  cephalic 
and  ventral  surfaces.  Soon,  dorsal  prolongations  of  the  lateral  liver 
lobes,  the  coronary  appendages,  come  into  relation  with  the  diaphragm 
dorsally  and  laterally  (Fig.  124).  The  attachment  of  the  liver  to  the 
septum  transversum  now  has  the  form  of  a crescent,  the  dorsal  horns  of 
which  are  the  coronary  appendages  (Fig.  113).  This  union  becomes  the 
coronary  ligament  of  the  adult  liver.  The  dorso-ventral  extent  of  the 
coronary  ligament  is  reduced  during  development,  and,  at  five  months,  its 
lateral  extensions  upon  the  diaphragm  give  rise  to  the  triangular  ligaments 
of  each  side. 

The  right  lobe  of  the  liver,  comes  into  relation  along  its  dorsal  sur- 
face with  the  caval  mesentery  of  9 mm.  embryos  (Figs.  112  and  113). 
This  attachment  extends  the  coronary  ligament  caudally  on  the  right  side 
and  makes  possible  the  connection  between  the  veins  of  the  liver  and 
mesonephros  which  contributes  to  the  formation  of  the  inferior  vena 
cava.  The  portion  of  the  liver  included  between  the  caval  mesentery 
and  the  lesser  omentum  is  the  caudate  lobe.  The  umbilical  vein  (later 
the  ligamentum  teres)  courses  in  a deep  groove  along  the  ventral  surface 
of  the  liver,  and,  with  the  portal  vein  and  gall  bladder,  bounds  the  quad- 
rate lobe. 

In  general,  the  several  displacements  and  secondary  fusions  of  the 
primitive  mesentery  cause  its  line  of  peritoneal  attachment  to  depart 
throughout  most  of  its  extent  from  the  original  midsagittal  position. 


THE  CCELOM 


127 


The  special  mesenterial  supports  of  the  urogenital  organs  will  be  described 
in  the  next  chapter. 


Anomalies. — The  mesenteries  may  show  malformations,  due  to  the  persistence  of  the 
simpler  embryonic  conditions,  usually  correlated  with  the  defective  development  of  the 
intestinal  canal.  In  about  30  per  cent  of  cases  the  ascending  and  descending  mesocolon 
are  more  or  less  free,  having  failed  to  fuse  with  the  dorsal  peritoneum.  The  primary 
sheets  of  the  greater  omentum  may  also  fail  to  unite,  so  that  the  inferior  recess  extends  to 
the  caudal  end  of  the  greater  omentum,  as  is  normal  in  many  mammals. 

II.  THE  CCELOM 


Pericardial  cavity 
Surface  of  fore-gut 


The  Primitive  Coelom.— The  first  occurrence  of  a coelom  is  in  the 
extra-embryonic  mesoderm  (Fig.  40  C).  vShortly  after,  numerous  clefts 
appear  in  the  embryonic  mesoderm  of  each  side  and  split  it  into  somatic 
and  splanchnic  layers  (cf.  Fig.  325). 

These  clefts  coalesce  in  the  cardiac  region 
and  form  two  elongated  pericardial  cav- 
ities, lateral  to  the  paired  heart  tubes  (Fig. 

165  A,  B).  Similarly,  right  and  left 
pleuro-peritoneal  cavities  are  formed  be- 
tween the  mesoderm  layers  caudal  to  the 
heart.  The  paired  pericardial  cavities 
extend  toward  the  midplane  cranial  to 
the  heart  and  presently  communicate 
with  each  other  (Figs.  117  and  165  C). 

Laterally,  they  are  not  continuous  with 
the  extra-embryonic  coelom,  for  in  this 
region  the  head  of  the  embryo  has  already 
separated  from  the  underlying  blastoderm . 

The  pericardial  cavities,  nevertheless, 

are  prolonged  caudally  until  they  open  into  the  pleuro-peritoneal  cavities 
where  these  in  turn  communicate  laterally  with  the  extra-embryonic 
coelom.  In  an  embryo  of  2 mm.,  the  coelom  thus  consists  of  a U-shaped 
pericardial  cavity,  the  right  and  left  limbs  of  which  are  continued  caudally 
into  the  paired  pleuro-peritoneal  cavities;  these  extend  out  into  the  extra- 
embryonic  coelom  (Fig.  117). 

The  primitive  coelom  lies  in  the  horizontal  plane  (Fig.  117).  Coinci- 
dent with  the  caudal  regression  of  the  primitive  diaphragm,  the  peri- 
cardial cavity  is  bent  ventrad  and  enlarged  (Fig.  118).  The  ventral 
mesocardium,  attaching  the  heart  to  the  ventral  body  wall,  disappears, 
and  the  right  and  left  limbs  of  the  U-shaped  cavity  become  confluent, 
ventral  to  the  heart.  The  result  is  a single,  large  pericardial  chamber, 
the  long  axis  of  which  now  lies  in  a dorso-ventral  plane,  nearly  at  right 
angles  to  the  plane  of  the  pleuro-peritoneal  cavities,  and  connected  with 


Pleuro-peritoneal  canal 
Entoderm  of  gut 

Peritoneal  cavity 
^Extra-embryonic  ccelom 
Wall  of  yolk  sac 
Fig.  II 7. — Diagrammatic  model  of  the 
fore-gut  and  coelom  in  an  early  human 
embryo,  viewed  from  above  and  behind 
(Robinson-Prentiss) . 


128 


THE  MESENTERIES  AND  CCELOM 


them  dorsally  by  the  right  and  left  pleuro-pericardial  canals.  On  account 
of  the  more  rapid  growth  of  the  embryo,  there  is  an  apparent  constric- 
tion at  the  yolk  stalk,  and,  with  the  development  of  the  umbilical  cord, 
the  peritoneal  cavity  is  separated  definitely  from  the  extra-embryonic 
coelom  (Fig.  45).  Dorsally,  the  pleural  and  peritoneal  cavities  are  per- 
manently partitioned  lengthwise  by  the  dorsal  mesentery. 

1 he  cavities  of  the  mesodermal  segments  are  regarded  as  portions  of 
the  coelom,  but  in  man  they  disappear  early.  The  development  of  the 
vaginal  sacs,  which  grow  out  from  the  inguinal  region  of  the  peritoneal 
cavity  into  the  scrotum,  will  be  described  in  Chapter  VIII. 

1 he  division  of  the  primitive  coelom  into  separate  cavities  is  accom- 
plished by  the  development  of  three  types  of  membrane  that  join  on  each 


Fig.  1 18. — Reconstruction,  cut  at  the  left  of  the  median  sagittal  plane  of  a 3 mm.  human  em- 
hryo,  showing  the  body  cavities  and  septum  transversum  (Kollmann). 

side  in  a Y-shaped  fashion  (Figs.  122  and  123);  (i)  the  unpaired  septum 
transversum,  which  separates  partially  the  pericardial  and  pleural  cavities 
from  the  peritoneal  cavity;  (2)  the  paired  pleuro-pericardial  membranes, 
which  complete  the  division  between  pericardial  and  pleural  cavities; 
(3)  the  paired  pleuro- peritoneal  membranes,  which  complete  the  partition 
between  each  pleural  cavity  and  the  peritoneal  cavity. 

The  Septum  Transversum. — The  vitelline  veins,  on  their  way  to  the 
heart,  course  in  the  splanchnic  mesoderm  lateral  to  the  fore-gut  (Fig. 
183).  In  embryos  of  2 to  3 mm.,  these  large  vessels  bulge  into  the  coelom 
until  they  meet  and  fuse  with  the  somatic  mesoderm  (Fig.  381).  Thus, 


THE  CCELOM 


I2Q 


there  is  formed  caudal  to  the  heart  a transverse  partition,  filling  the  space 
between  the  sinus  venosus  of  the  heart,  the  gut,  and  the  ventral  body 
wall,  and  separating  the  pericardial  and  peritoneal  cavities  from  each 
other  ventral  to  the  gut  (Fig.  i66).  This  mesodermal  partition  was 
termed  by  His  the  septum  transversum.  In  Fig.  ii8  it  comprises  both  a 
cranial  portion  (designated  “septum  transversum”)  that  is  the  anlage 
of  a large  part  of  the  diaphragm,  and  a 
caudal  portion,  the  ventral  mesentery, 
into  which  the  liver  is  growing. 

At  first  the  septum  transversum 
does  not  extend  dorsal  to  the  gut,  but 
leaves  on  either  side  a pleura- peritoneal 
canal  through  which  the  pericardial  and 
pleuro-peritoneal  cavities  communicate 
(Fig.  1 1 8).  In  embryos  of  4 to  5 mm., 
the  lungs  develop  in  the  median  walls 
of  these  canals  and  bulge  laterally  into 
them  (Fig.  120).  Thus  the  canals  be- 
come the  pleural  cavities,  and  will  be  so 
termed  hereafter. 

The  septum  transversum  of  2 mm. 
embryos  occupies  a transverse  position 
in  the  middle  cervical  region  (Fig.  119, 

2).  It  then  migrates  caudally,  the 
ventral  portion  at  first  moving  more 
rapidly  so  that  its  position  becomes 
oblique.  In  5 mm.  embryos  (Fig.  119, 

5)  the  septum  is  opposite  the  fifth 
cervical  segment,  at  which  level  it  re- 
ceives the  phrenic  nerve.  During  a 
second  period  of  migration  the  dorsal 
attachment  travels  faster  than  the 
ventral  portion,  and  as  a result  the  septum  rotates  to  a position  nearly  at 
right  angles  to  its  plane  at  7 mm.  The  final  location,  opposite  the  first 
lumbar  segments,  is  attained  in  an  embryo  of  two  months. 

The  Pleuro-pericardial  and  Pleuro-peritoneal  Membranes. — The 
common  cardinal  veins  (ducts  of  Cuvier),  on  their  way  to  the  heart,  curve 
around  the  pleural  cavities  laterally  in  the  somatic  body  wall  (Fig.  118). 
In  embryos  of  7 mm.,  each  vein,  with  the  overlying  mesoderm,  forms  a 
ridge  that  projects  from  the  body  wall  mesially  into  the  adjacent  pleural 
canal.  This  ridge,  the  pulmonary  ridge  (of  Mall),  is  the  anlage  of  both 
the  pleuro-pericardial  and  pleuro-peritoneal  membrane  (Figs.  118  and 


Fig.  1 19. — Diagram  showing  the  migra- 
tion of  the  septum  transversum  (Mall- 
Prentiss).  Numerals  indicate  the  length 
of  the  embryo  at  each  position  of  the 
septum.  The  letters  and  numbers  at  the 
right  represent  the  occipital,  cervical, 
thoracic  and  lumbar  segments. 


THE  MESENTERIES  AND  CCELOM 


130 


Fig.  120. — Reconstruction  of  a 7.5  mm.  human  embryo,  cut  across  and  viewed  caudally  to 
show  the  body  cavities  and  pulmonary  ridge  (Kollman). 


Pericardial  cavity  Common  cardinal  vein 


Stomach 


Mesonephros 


Pleuro- pericardial 


Lung 


Pleuro- 

peritoneal 

membrane 


Vein  to  limb 
bud 


Fig.  121. — Reconstruction  of  a 7 mm.  human  embryo,  showing  from  the  left  side  the 
pleuro-pericardial  membrane,  the  pleuro-peritoneal  membrane  and  the  septum  transversum 
(Mall  in  Prentiss).  X 20.  The  phrenic  nerve  courses  in  the  pleuro-pericardial  membrane. 
An  arrow  jiasses  from  pericardial  to  peritoneal  cavity  through  the  pleuro-pericardial  canal. 


THE  COELOM 


I3I 

120).  Later,  it  broadens  and  thickens  cranio-caudally  (Fig.  121),  forming 
a triangular  structure  whose  apex  is  continuous  with  the  septum  trans- 
versum  (Fig.  122).  Its  cranial  side  constitutes  the  pleuro-pericardial 
membrane,  and,  in  g to  10  mm.  embryos,  reduces  the  opening  between  the 
pleural  and  pericardial  cavities  to  a mere  slit.  Its  caudal  side  becomes 
the  pleuro-peritoneal  membrane,  which  later  completes  the  partition  dorsally 
between  the  pleural  and  peritoneal  cavities  (Fig.  123). 


Pleuro-pericardial  membrane  Phrenic  nerve 


Fig.  122. — Reconstruction  of  an  1 1 mm.  human  embryo,  to  show  the  structures  of  Fig.  121  at 
a later  stage  (Mall  in  Prentiss).  X 14. 

The  two  sets  of  membranes  at  first  lie  nearly  in  the  sagittal  plane, 
and  a portion  of  each  lung  is  caudal  to  the  corresponding  pleuro-peri- 
toneal membrane  (Fig.  121).  Between  the  stages  of  7 and  ii  mm.  the 
dorsal  attachment  of  the  septum  transversum  shifts  caudad  more  rapidly 
than  its  ventral  portion,  and  carries  the  pleuro-peritoneal  membrane  with 
it  until  the  latter  lies  caudad  to  the  lung  (Figs,  iig,  121  and  122).  Each 
lung  then  occupies  a spherical  triangle  between  pleuro-pericardial  and 
pleuro-peritoneal  membranes  (Fig  122).  During  this  rotation  the  dorsal 
end  of  the  pleuro-pericardial  membrane  lags  behind,  anchored  by  the 


132 


THE  MESENTERIES  AND  CCELOM 


phrenic  nerve  which  courses  through  it,  and  so  takes  up  a position  in  a 
coronal  plane  nearly  at  right  angles  to  the  septum  transversum  (Figs. 
122  and  123).  In  1 1 mm.  embryos,  the  pleuro-pericar dial  membranes  have 
fused  completely  on  each  side  with  the  median  walls  of  the  pleural  cavi- 
ties (Fig.  123). 

The  pleuro-peritoneal  membranes  are  continuous  dorsally  and  caud- 
ally  with  the  mesonephric  folds;  ventrally  and  caudally,  they  fuse  later 
with  the  dorsal  pillars  of  the  diaphragm,  or  coronary  appendages  of  the 


Fig.  123. — Transverse  section  through  a 10  mm.  human  embryo,  showing  the  pleuro-pericardial 
and  pleuro-peritoneal  membranes  (Prentiss).  X 33. 

liver  (Figs.  113  and  124).  Between  the  free  margins  of  the  membranes 
and  the  mesentery  a temporary  opening  is  left  on  each  side,  through  which 
the  pleural  and  peritoneal  cavities  communicate  (Figs.  108,  113  and  122). 
Owing  to  the  caudal  migration  of  the  septum  transversum  and  the  growth 
of  the  lungs  and  liver,  the  pleuro-peritoneal  membrane,  at  first  lying  in 
a nearly  sagittal  plane  (Figs.  120  and  12 1),  is  shifted  to  a horizontal 
position  (Fig.  122),  and  gradually  its  free  margin  unites  with  the  dorsal 
pillars  of  the  diaphragm  and  with  the  dorsal  mesentery.  The  opening 
between  the  pleural  and  peritoneal  cavities  is  thus  narrowed  and  finally 
closed  in  embryos  of  19  to  20  mm. 

The  Pericardium  and  Diaphragm. -The  lungs  grow  and  expand, 
not  only  cranially  and  caudally  but  also  laterally  and  ventrally  (Fig. 
125).  Room  is  made  for  them  by  the  obliteration  of  the  very  loose, 
spongy  mesenchyme  of  the  adjacent  body  wall  (Fig.  124).  As  the  lungs 


THE  CCELOM 


133 


burrow  laterally  and  ventrally  into  the  body  wall  around  the  pericardial 
cavity,  the  pleuro-pericardial  membranes  enlarge  at  the  expense  of  this 
tissue  and  more  and  more  the  heart  comes  to  lie  in  a mesial  position 


Fig.  124. — Transverse  section  through  a 10  mm.  human  embryo,  showing  the  fusion  of  the 
pleuro-peritoneal  membranes  with  the  coronary  appendages  (Prentiss).  X 16. 

between  the  lungs,  but  separated  from  them  by  the  pericardium  (Fig. 
125  B).  The  pleural  cavities  thus  increase  rapidly  in  size. 


Esophagus 
Right  lung 

Coronary  appendage  of 
liver 

Vena  cava  inferior 


Pleural  cavity 


peritoneal 
me}7ibrane 


hrenic  jierve  in  septum 
transversum 


A 

Esophagus 


B 


Pericardial  cavity 


Lung 

Pleural  cavity 
Heart  Pericardium 

Fig,  125. — Diagrams  showing  the  development  of  the  lungs  and  the  formation  of  the  pericar- 
dium (Robinson-Prentiss).  A,  Coronal  section;  B,  transverse  section. 


At  the  same  time  the  liver  grows  enormously,  and  on  either  side  a 
portion  of  the  body  wall  is  taken  up  into  the  septum  transversum  and 


134 


THE  MESENTERIES  AND  CCELOM 


pleuro-peritoneal  membranes.  The  diaphragm,  according  to  Broman, 
is  thus  derived  from  four  sources  (Fig.  126):  (i)  its  ventral  pericardial 
portion  from  the  septum  transversum;  its  lateral  portions  from  (2)  the 
pleuro-peritoneal  membranes,  plus  (3)  derivatives  from  the  body  wall; 
(4)  lastly,  a median  dorsal  portion  is  formed  from  the  dorsal  mesentery. 
In  addition  to  these,  the  striated  muscle  of  the  diaphragm  takes  its  origin 
from  a pair  of  premuscle  masses,  which,  in  9 mm.  embryos,  lie  one  on  each 
side  opposite  the  fifth  cervical  segment  (Bardeen).  This  is  the  level  at 


Fig.  126. — Diagram  showing  the  contributions  to  the  diaphragm  (Broman).  i.  Septum 
transversum;  2,  3,  dorsal  mesentery;  4,  4,  pleuro-peritoneal  membranes;  5,  5,  bodj'  wall;  A, 
aorta;  Oe,  esophagus;  VC,  inferior  vena  cava. 

which  the  phrenic  nerve  enters  the  septum  transversum  (Fig.  124).  The 
exact  origin  of  these  muscle  anlages  is  in  doubt,  but  they  probably  repre- 
sent portions  of  the  cervical  myotomes  of  this  region.  The  muscle  masses 
migrate  caudally  with  the  septum  transversum  and  develop  chiefly  in 
the  dorsal  portion  of  the  diaphragm. 

Anomalies. — The  persistence  of  a dorsal  opening  in  the  diaphragm,  more  commonly 
on  the  left  side,  finds  its  explanation  in  the  imperfect  development  of  the  pleuro-peritoneal 
membrane.  Such  a defect  may  lead  to  diaphragmatic  hernia,  the  abdominal  viscera  pro- 
jecting to  a greater  or  less  extent  into  the  pleural  cavity.  Similarly,  faulty  development 
of  the  left  pleuro-pericardial  membrane  sometimes  causes  the  heart  and  left  lung  to  occupy 
a common  cavity.  An  intact  diaphragm,  locally  deficient  in  muscle,  may  herniate. 


CHAPTER  VIII 


THE  UROGENITAL  SYSTEM 

The  urinary  and  reproductive  systems  are  associated  intimately 
in  development.  Both  arise  from  the  mesoderm  of  the  intermediate 
cell  mass  fnephrotome),  which  unites  the  primitive  segments  with  the 
lateral  layers  of  somatic  and  splanchnic  mesoderm  (Figs.  35  S and  128). 
In  the  course  of  development  these  anlages  bulge  into  the  coelom  as  paired 
longitudinal  ridges,  termed  the  urogenital  folds  (Figs.  131,  143  and  144). 

Vertebrates  possess  excretory  organs  of  three  district  types.  The 
pronephros  is  the  functional  kidney  of  amphioxus  and  certain  lampreys, 
but  appears  in  immature  fishes  and  amphibians  only  to  be  replaced  by  the 
mesonephros.  The  embryos  of  amniotes  (reptiles,  birds,  and  mammals) 
develop  first  a pronephros,  and  then  a mesonephros,  whereas  the  permanent 
kidney  is  a new  organ,  the  metanephros.  Whether  these  glands  represent 
modifications  of  an  originally  continuous  organ,  or  whether  they  are 
three  distinct  structures,  is  undecided,  but  however  this  may  be  the  pro- 
meso-,  and  metanephroi  of  amniotes  develop  successively,  one  caudad  of 
the  other,  in  the  order  named. 

I.  THE  URINARY  ORGANS 

The  Pronephros. — When  functional,  the  pronephros  consists  of  paired, 
segmentally  arranged  pronephric  tubules;  one  end  of  each  tubule  opens 
into  the  coelom,  the  other  into  a longitudinal  collecting  duct  which  drains 
into  the  cloaca  (Fig.  128  A).  Near  the  nephrostome  (the  funnel-like  open- 
ing into  the  coelom),  knots  of  arteries  project  into  the  coelom,  forming 
glomeruli.  These  filter  wastes  from  the  blood  into  the  coelomic  fluid 
which  is  then  taken  up  by  the  tubules  and  carried  by  ciliary  movement  into 
the  excretory  ducts. 

The  human  pronephros  is  vestigial.  It  consists  of  about  seven  pairs 
of  rudimentary  pronephric  tubules,  formed  as  dorsal  sprouts  from  the 
nephrotomes  in  each  segment,  from  the  seventh  to  the  fourteenth,  and 
perhaps  from  more  cranial  segments  as  well  (Fig.  35  B).  Y"et  the  earliest 
tubules  begin  to  degenerate  before  the  last  appear.  The  nodules  hollow 
out  and  open  into  the  coelom  (Fig.  128  B).  Dorsally  and  laterally,  the 
tubules  of  each  side  bend  backward  and  unite  to  form  a longitudinal 
pronephric  collecting  duct  (Fig.  12S  B,  A).  Caudal  to  the  fourteenth  seg- 
ment no  pronephric  tubules  are  developed,  but  the  free  end  of  the  collect- 

135 


136 


THE  UROGENITAL  SYSTEM 


Fig.  127. — Transverse  section  of  a 2.4  mm.  human  embryo,  showing  the  intermediate  cell  mass 

or  nephrotome  (Kollmann). 


A Proncphric  duel  B 


Anlages  of  proncphric 
tubules  and  duct 


Nephrotome 
Somatic  mesoderm 


Splanchnic  mesoderm 
Notoehord  Entoderm 


Fig.  128. — Diagrams  illustrating  the  development  of  the  proncphric  duct  and  proncphric 
tubules  (Felix-Prentiss).  A,  represents  a later  stage  than  B. 


THE  URINARY  ORGANS 


137 


ing  duct,  by  a process  of  terminal  growth,  extends  caudad  beneath  the 
ectoderm  (and  lateral  to  the  nephrogenic  cord)  until  it  reaches  the  lateral 
wall  of  the  cloaca  and  perforates  it  (Fig.  87).  Thus  are  formed  the  paired 
primary  excretory  {prone phric)  ducts.  The  pronephric  tubules  begin  to 
appear  in  embryos  of  1.7  mm.,  with  nine  or  ten  primitive  segments;  at  2.5 
mm.  (23  segments),  all  the  tubules  have  developed  and  the  primary  excre- 
tory duct  is  nearly  complete.  In  4.3  mm.  embryos,  the  duct  has  reached 
the  wall  of  the  cloaca  and  soon  after  fuses  with  it  (Fig.  87).  The  pro- 
nephric tubules  promptly  degenerate,  but  the  primary  excretory  ducts 
persist  and  become  the  ducts  of  the  mesonephroi. 

The  Mesonephros. — The  mesonephros,  like  the  pronephros,  consists 
essentially  of  a series  of  tubules,  each  of  which  at  one  end  is  related  to  a 
knot  of  blood  vessels  and  at  the  other  end  opens  into  the  primary  excretory 
duct  (Fig.  87).  But  the  mesonephric  tubule  differs  in  two  important 
respects:  ( i)  the  glomerulus  indents  one  end  of  the  tubule,  and  excreta  from 
the  blood  pass  directly  into  its  lumen;  (2)  the  nephrostomes  are  transitory 
and  never  open  into  the  mesonephric  chamber.  The  mesonephric  tubules 
arise  just  caudal  to  the  pronephros  and  from  the  same  general  source,  that 
is,  the  nephrotomes.  Only  a few  of  the  more  cranial  tubules,  however,  are 
formed  from  distinct  intermediate  cell  masses,  for,  caudal  to  the  tenth  pair 
of  segments,  this  mesoderm  fuses  into  unsegmented,  paired  nephrogenic 
cords  which  may  extend  as  far  as  the  twenty-eighth  segment  (Fig.  132). 
The  primary  excretory  ducts  lie  lateral  to  the  nephrogenic  cords. 

When  the  developing  mesonephric  tubules  begin  to  expand,  there  is 
not  room  for  them  in  the  dorsal  body  wall,  which  as  a result  bulges  ven- 
trally  into  the  eoelom.  Thus  is  produced  on  each  side  of  the  dorsal 
mesentery  a longitudinal  urogenital  fold,  which  may  extend  from  the  sixth 
cervical  to  the  third  lumbar  segment  (Fig.  143).  Later,  this  ridge  is 
divided  into  a lateral  mesonephric  fold  and  into  a median  genital  fold,  the 
anlage  of  the  genital  gland  (Figs.  13 1 and  144). 

Differentiation  of  the  Tidmlcs. — The  nephrogenic  cord  in  2.5  mm. 
embryos  first  divides  into  spherical  masses  of  cells,  the  anlages  of  the 
mesonephric  tubules.  As  many  as  four  of  these  are  formed  in  a single 
segment.  Appearing  first  in  the  thirteenth  to  fifteenth  segments,  the 
anlages  of  the  tubules  differentiate  both  cranially  and  caudally.  In  5.3 
mm.  embryos  the  cephalic  limit  is  reached  in  the  sixth  cervical  segment, 
and  thereafter  degeneration  begins  at  this  end  (Fig.  130).  Hence,  the 
more  cranial  tubules  overlap  those  of  the  pronephros.  In  7 mm.  embryos, 
the  caudal  limit  is  reached  in  the  third  lumbar  segment. 

Differentiation  of  the  tubule  anlages  progresses  in  a cephalo-caudad 
direction  (Fig.  130).  First,  ve.sicles  with  lumina  are  formed  (4.3  mm.) 
from  the  spherical  masses  (Fig.  i2g  A.B).  Next,  the  vesicles  elongate  later- 


138 


THE  UROGENITAL  SYSTEM 


ally,  unite  with  the  primary  excretory  ducts,  and  become  S-shaped  (Fig. 
129  B,  C).  The  free,  vesicular  end  of  the  tubule  enlarges,  becomes  thin 
walled,  and  into  this  wall  grows  a knot  of  arteries  to  form  the  glomerulus 
(5  to  7 mm. ; Fig.  129  D).  The  wall  of  the  vesicle  about  the  glomerulus  is 
Bowman  's  capsule  and  the  two  constitute  a renal  corpuscle  of  the  mesone- 
phros (Fig.  131).  The  tubule,  which  was  at  first  solid,  is  now  lined  with  a 
low  columnar  epithelium.  In  the  human  embryo  the  tubules  do  not 
branch  or  coil  as  in  the  pig,  consequently  the  mesonephros  is  relatively 
smaller.  At  10  mm.,  about  35  tubules  are  present  in  each  mesonephros 


Mesonephric  duct 


Anldge  of 
mesonephric  luhnlc 


C 

Mesonephric  duct 


Fig.  i2y. — Diagrams  showing  the  differ-  Fig.  130. — The  anlages  of  the  urinary  or- 

entiation  of  a mesonephric  tubule  (Felix-  gans  in  a 10  mm.  human  embryo,  as  seen 
Prentiss).  L.  lateral;  M.  median.  from  the  left  side  (adapted  by  Prentiss). 


and  the  glomeruli  are  conspicuous  (Fig.  130).  Each  tubule  shows  a 
distal  secretory  portion  and  a proximal  collecting  part  which  connects 
with  the  duct  (Fig.  13 1).  The  glomeruli  form  a single  median  column  in 
the  gland;  the  tubules  are  dorsal  and  the  duct  is  lateral  in  position. 
Ventro-lateral  branches  from  the  aorta  supply  the  glomeruli  (Fig.  212), 
while  the  posterior  cardinal  veins  (Fig.  145),  dorsal  in  position,  break  up 
into  a network  of  sinusoids  about  the  tubules. 


THE  URINARY  ORGANS 


139 


The  pronephric  duct,  now  termed  the  mesonephric,  or  Wolffian  duct,  is 
solid  in  4.3  mm.  embryos.  A lumen  is  formed  at  7 mm.,  wider  opposite 
the  openings  of  the  tubules.  The  duct  is  important,  as  the  ureteric  anlage 
of  the  permanent  kidney  grows  out  from  its  caudal  end,  while  the  tube 
itself  is  transformed  into  the  chief  genital  duct  of  the  male. 

That  the  human  mesonephros  is  a functional  excretory  organ  is 
plausible  (Bremer,  1916),  but  not  proved.  Degeneration  proceeds  rapidly 
in  embryos  between  10  and  20  mm.  long,  beginning  cranially  (Fig.  148). 
New  tubules  are  formed  at  the  same  time  caudally  (Fig.  130).  In  all. 


83  pairs  of  tubules  arise,  of  which  only  26  pairs  persist  at  21  mm.,  and  these 
are  usually  broken  at  the  angle  between  the  collecting  and  secretory 
regions.  How  the  genital  system  utilizes  them  for  new  purposes  will  be 
traced  in  a later  section  (p.  156). 

The  Metanephros. — The  essential  parts  of  the  permanent  kidney 
are  the  renal  corpuscles  (glomeruli  with  Bow^man’s  capsules),  secretory 
tubules,  and  collecting  tubules.  Like  the  mesonephros,  the  metanephros  is 
of  double  origin.  The  ureter,  pelvis,  calyces,  and  collecting  tubules  are 
outgrowths  of  the  mesonephric  duct.  The  secretory  tubules  and  the  cap- 
sules of  the  renal  corpuscles  are  differentiated  from  the  isolated,  caudal 
end  of  the  nephrogenic  cord  and  thus  have  an  origin  similar  to  that  of  the 
mesonephric  tubules. 

In  embryos  of  about  5 mm.  the  mesonephric  duct  makes  a sharp  bend 
just  before  it  joins  the  cloaca,  and  it  is  at  this  angle  that  the  ureteric 
evagination  appears,  dorsal  and  somewhat  median  in  position  (Fig.  139 
B,  C).  The  bud  grows  at  first  dorsally,  then  cephalad.  Its  distal  end 
expands  and  forms  the  primitive  pelvis;  its  proximal  elongated  portion  is 
the  ureter.  The  pelvic  anlage  grows  into  the  lower  end  of  the  nephro- 


140 


THE  UROGENITAL  SYSTEM 


genic  cord  (Fig.  132),  which,  during  the  third  month,  becomes  separated 
from  the  cranial  end.  The  nephrogenic  tissue  forms  a cap  about  the 
primitive  pelvis,  and,  as  the  pelvis  grows  cranially,  is  carried  along  with  it. 


Fig.  132. — Reconstruction  of  the  metancphric  anlages  in  a human  embryo  of  about  9 mm- 

(after  Schreiner). 


In  embryos  of  g to  13  mm.  the  pelvis,  having  advanced  cephalad  through 
three  segments,  attains  a position  in  the  retroperitoneal  tissue  dorsal  to 


Fig.  133. — Diagrams  showing  the  development  of  the  primitive  pelvis,  calyces  and  collecting 
tubules  of  the  metanephros  (adapted  by  Prentiss). 


the  mesonephros  and  opposite  the  second  lumbar  segment.  Thereafter, 
the  kidney  enlarges  both  cranially  and  caudally  without  shifting  its  mean 
position  (Fig.  1 54). 


THE  URINARY  ORGANS 


I4I 

Differentiation  of  the  Ureteric  Anlage. — Primary  collecting  tubules 
grow  out  from  the  primitive  pelvis  in  10  mm.  embryos.  Of  the  first  two, 
one  is  cranial,  the  other  caudal  in  position,  and,  between  these,  two  others 
usually  appear  (Fig.  133  B,  C).  From  an  ampullary  enlargement,  at  the 
end  of  each  primary  tubule  sprout  off  two,  three,  or  four  secondary  tubules. 
These  in  turn  give  rise  to  tertiary  tubules  (Fig.  133  D)  and 
repeated  until  the  fifth  month  of  fetal  life,  when  it  is  estimated 
that  twelve  generations  of  tubules  have  been  developed. 

The  pelvis  and  the  primary  and  secondary  tubules  enlarge 
greatly  during  development.  The  two  primary  expansions 
become  the  major  calyces,  and  the  secondary  tubules  opening 
into  them  form  the  minor  calyces  (Fig.  134).  The  tubules 
of  the  third  and  fourth  orders  are  taken  up  into  the  walls 
of  the  enlarged  secondary  tubules  so  that  the  tubules  of  the 
fifth  order,  20  to  30  in  number,  open  into  the  minor  calyces 
as  papillary  ducts.  The  remaining  orders  of  tubules  con- 
stitute the  collecting  tubules  which  form  the  greater  part  of 
the  medulla  of  the  adult  kidney. 

When  the  four  to  six  primary  tubules  develop,  the 
nephrogenic  cap  about  the  primitive  pelvis  is  subdivided 
and  its  four  to  six  parts  cover  the  end  of  each  primary  tubule. 

As  new  orders  of  tubules  arise,  each  mass  of  nephrogenic 
tissue  increases  in  amount  and  is  further  subdivided  until 
finally  it  forms  a peripheral  layer  about  the  tips  of  the 
branches  tributary  to  a primary  tubule.  The  converging 
branches  of  such  a tubular  ‘tree’  constitute  a primary  renal 
unit,  or  pyramid,  with  its  base  at  the  periphery  of  the  kidney 
and  its  apex  projecting  pnto  the  pelvis.  The  apices  of  the  pyramids  are 
termed  renal  papilla:,  and  through  them  the  papillary  ducts  open.  The 
nephrogenic  tissue  forms  the  cortex  of  the  kidney,  and  each  subdivision 
of  it,  covering  the  tubules  of  a pyramid  peripherally,  is  marked 
off  on  the  surface  of  the  organ  by  grooves  or  depressions.  The 
human  fetal  kidney  is  thus  distinctly  lobed,  the  lobations  persisting 
for  several  years  after  birth;  this  condition  is  permanent  in  reptiles, 
birds,  and  some  mammals  (whale;  bear;  ox).  The  primary  p^’ramids 
are  subdivided  into  several  secondary  and  tertiary  pyramids.  Between 
the  pyramids,  the  cortex  of  nephrogenic  tissue  dips  down  to  the  pelvis, 
forming  the  renal  columns  (of  Bertin).  The  collecting  tubules,  on 
the  other  hand,  extend  out  into  the  cortex  as  the  cortical  rays,  or  pars 
radiata  of  the  cortex.  In  these  rays,  and  in  the  medulla  of  the  kidney,  the 
collecting  tubules  run  parallel  and  converge  to  the  papillae. 


the  process  is 


Fig.  I 3 4 • — 
Reconstruc  t i o n 
of  the  ureter, 
pelvis,  calyces 
and  their  bran- 
ches from  a 16 
mm.  human 
embryo  (Huber). 
X 50. 


142 


THE  UROGENITAL  SYSTEM 


Differentiation  of  the  Nephrogenic  Tissue. — In  stages  from  13  to  19 
mm.,  the  nephrogenic  tissue  about  the  ends  of  the  collecting  tubules 
condenses  into  spherical  masses  that  lie  in  the  angles  between  the  buds  of 
new  collecting  tubules  and  their  parent  stems  (Fig.  135).  One  such 
metanephric  sphere  is  formed  for  each  new  tubule.  The  spheres  are  con- 
verted into  vesicles  with  eccentrically  jilaced  lumina.  The  vesicle  elon- 
gates, its  thicker  outer  wall  forming  an  S-shaped  tubule  which  unites  with 
a collecting  tubule,  its  thin  inner  wall  becoming  the  capsule  (Bowman’s) 
of  a renal  corpuscle. 

The  uriniferous  tubules  of  the  adult  kidney  have  a definite  and 
peculiar  structure  and  arrangement  (Fig.  136  4l).  Beginning  with  a renal 


(Huber).  The  left  half  of  each  figure  represents  an  earlier  stage  than  the  right  half. 

corpuscle,  each  tubule  forms  a proximal  convoluted  portion,  a \J-shaped 
loop  (of  Henle)  with  descending  and  ascending  limbs,  a connecting  piece, 
which  lies  close  to  the  renal  corpuscle,  and  a distal  convoluted  portion 
continuous  with  the  collecting  tubule.  These  parts  are  derived  from  the 
S-shaped  anlage,  which  is  composed  of  a lower,  middle,  and  upper  limb. 
The  middle  limb,  somewhat  U-shaped,  bulges  into  the  concavity  of  Bow- 
man’s capsule  (Fig.  136  B).  By  differentiation  the  lower  portion  of  the 
lower  limb  is  converted  into  Bowman’s  capsule,  and  ingrowing  arteries 
form  the  glomerulus  (Fig.  1^6  B,  C).  The  upper  part  of  the  same  limb  by 


THE  URINARY  ORGANS 


143 


enlargement,  elongation,  and  coiling  becomes  the  proximal  convoluted 
tubule.  The  neighboring  portion  of  the  middle  limb  forms  the  primitive 


Arch  of  collecting  tubule 
A 


Proximal  convoluted 
tubule 

Distal  convoluted, 
tubule 

Renal  -corpuscle- 
Connecting  piece~^ 


Ascending  limb  of. 
Uenle’s  loop 


Descending  limb  of. 
Henle's  loop 

Large  collecting, 
tubule 


Arch  of  collecting  tubule 

Distal  convoluted  tubule 
Stoerck’s  loop 
Proximal  convoluted  tubule 

Connecting  piece 
Glomerulus 

Boic'ma}i’s  capsule 


Arch  of  collecting  tubide 

Proximal  convoluted  tubule 

Distal  convoluted  tubule 
Connecting  piece 
Glomerulus 

Bowman's  capsule 
Stoerck's  loop 


Fig.  136. — Diagrams  showing  the  differentiation  of  the  various  parts  of  a human  uriniferous 
tubule  (adapted  by  Prentiss).  A,  From  an  adult;  B,  C,  from  embryos. 


Fig.  137. — Diagram  showing  the  relation  of  Bowman’s  capsule  and  the  uriniferous  tubule 
to  the  collecting  tubules  of  the  metanephros  (Huber),  c,  Collecting  tubule;  e,  end  branches  of 
collecting  tubules;  r,  renal  corpuscles;  n,  neck;  pc,  pro.ximal  convoluted  tubule;  dl,al,  descending 
and  ascending  limbs  of  Henle’s  loop,  /;  dc,  distal  convoluted  tubule;  j,  junctional  tubule. 

loop  (of  Stoerck) ; the  base  of  the  middle  limb  gives  rise  to  the  connecting 
piece,  and  the  rest  of  it,  rvith  the  upper  limb  of  the  S,  comprises  the  distal 


144 


THE  UROGENITAL  SYSTEM 


convoluted  tuliule.  The  primitive  loop  of  Stoerck  includes  both  the 
descending  and  ascending  limbs  of  Henle’s  loop  and  a portion  of  the 
proximal  convoluted  tubule  as  well.  Henle’s  loop  is  differentiated  during 
the  fourth  fetal  month  and  extends  from  the  pars  radiata  of  the  cortex 
into  the  medulla  (Fig.  137).  The  concavity  of  Bowman’s  capsule,  into 
which  grow  the  arterial  loops  of  the  glomerulus,  is  at  first  shallow.  Even- 
tually, the  walls  of  the  capsule  grow  about  and  enclose  the  vascular  knot, 
except  at  the  point  where  the  arterioles  enter  and  emerge  (Fig.  135,4  and  5). 
Renal  corpuscles  are  first  fully  formed  at  the  end  of  the  second  month. 
The  newer  corpuscles  differentiate  peripherally  from  persisting  nephro- 


Fig.  138. — Reconstructed  stages  in  the  development  of  the  human  metanephric  tubule  at 

the  seventh  month  (Huber).  X 16. 

genic  tissue,  and  this  may  continue  for  some  time  after  birth;  hence,  in  the 
adult,  the  oldest  corpuscles  are  those  next  the  medulla.  Reconstructions 
of  the  various  stages  in  the  development  of  the  uriniferous  tubules  are 
shown  in  Fig.  138. 

Anomalies. — The  kidneys  may  fail  to  ascend  from  their  embryonic  position  in  the 
pelvis.  Absence  of  one  kidney  is  not  infrequent.  The  kidneys  sometimes  fuse,  either 
completely  into  a disc-shaped  mass,  or  partially  by  cortical  union  (V/cm’-i/zoe  ; 

in  such  cases  the  ducts  usually  are  bilateral.  Double  or  cleft  ureters  and  pelves  occur. 


THE  URINARY  ORGANS 


145 


Renal  C)^sts  (‘cystic  kidney ’)  result  from  the  primary  non-union  of  uriniferous  and  collect- 
ing tubules,  or  by  the  cystic  degeneration  of  secondarily  detached  tubules  (Kampmeier, 

1923)- 

Differentiation  of  the  Cloaca. — In  embryos  of  i . 4 mm.,  the  cloaca, 
a caudal  expansion  of  the  primitive  entodermal  canal,  is  in  contact  ven- 
trally  with  the  ectoderm,  and  the  area  of  union  constitutes  the  cloacal 
membrane  (Fig.  139  A).  This  membrane  at  first  extends  from  the  tail 
bud  to  the  body  stalk  (Figs.  71  and  95),  and  occupies  a region  corre- 
sponding to  the  hind  end  of  the  primitive  streak  (Figs.  44  and  58).  Later, 


Fig.  139. — Reconstructions  of  the  early  human  cloaca  (Pohlman-Prentiss).  X about  50.  .4 

3.5  mm.;  B,  4 mm.;  C,  5 mm.;  D,  7 mm. 

its  expanse  is  diminished  in  both  directions  (Figs.  96,  141  and  142). 
Ventro-cephalad,  the  cloaca  gives  off  the  allantoic  stalk,  receives  the 
mesonephric  ducts  laterally,  and  is  prolonged  caudally  as  the  tail-gut 
(Fig.  139  B). 

The  saddle-like  partition  between  the  intestine  and  allantois  grows 
caudally,  dividing  the  cloaca  into  a dorsal  rectum  and  ventral,  primitive 
urogenital  sinus  (Figs.  139  to  142).  The  division  is  complete  in  embr^'os 
of  II  to  15  mm.,  and  at  the  same  time  the  partition,  fusing  with 
the  cloacal  membrane,  divides  it  into  the  anal  membrane  of  the  gut  and 


10 


146 


THE  UROGENITAL  SYSTEM 


the  urogenital  membrane  (Fig.  142).  The  intermediate  tissue  represents  the 
body  of  the  primitive  perineum.  At  ii  mm.,  according  to  Felix,  the 
primitive  urogenital  sinus  by  elongation  and  constriction  is  differentiated 


Mesonephric  duct  Intestine 


Fig.  140. — Reconstruction  from  a 12  mm.  human  embryo,  showing  the  partial  division  of 
the  cloaca  into  rectum  and  urogenital  sinus  (Pohlman-Prentiss).  X 65. 


into  two  regions:  (i)  a dorsal  vesico-iirethral  anlage  which  receives  the 
allantois  and  mesonephric  duct,  and  is  connected  by  the  constriction  with 


Calom  Rectum 

Allantois 


Ureter 


Vesico-urcthral  anlage 
Phallic  portion  of  urogenital  sinus 


Metanephros 


Fig.  141. — Reconstruction  of  the  caudal  portion  of  an  11.5  mm.  human  embryo,  showing  the 
differentiating  rectum,  bladder  and  urethra  (Keibel-Prentiss).  X 25. 


(2)  the  phallic  portion  (Figs.  140  and  141).  The  latter  extends  into  the 
phallus  of  both  sexes  and  forms  a greater  part  of  the  male  urethra  (Fig. 
142),  as  described  on  p.  164. 


THE  URINARY  ORGANS 


147 


The  vesico-urethral  anlage  enlarges  and  transforms  into  the  bladder 
and  into  either  the  entire  female  urethra  or  the  prostatic  and  membranous 
male  urethra.  In  7 mm.  embryos  the  proximal  ends  of  the  mesonephric 
ducts  are  funnel  shaped,  and,  at  10  mm.,  eoincident  with  the  enlargement 
of  the  bladder,  these  ends  are  taken  up  into  its  wall  until  the  ureters  and 
mesonephric  ducts  acquire  separate  openings  (Figs.  141  and  142).  The 
ureters,  having  previously  shifted  their  openings  into  the  mesonephric 
ducts  from  a dorsal  to  lateral  position,  now  open  into  the  vesico-urethral 
anlage  lateral  to  the  mesonephric  ducts.  The  lateral  walls  of  the  bladder 
anlage  grow  more  rapidly  than  its  dorso-median  urethral  wall;  hence 
the  ureters  are  carried  cranially  and  laterally  upon  the  wall  of  the  bladder. 


Fig.  142. — Reconstructions  of  the  caudal  end  of  a two-months’  human  embryo,  showing 
the  complete  separation  of  the  rectum  and  urogenital  sinus  and  the  relations  of  the  urogenital 
ducts  (Keibel-Prentiss).  X 15. 

while  the  mesonephric  ducts  open  close  together  on  a hilloek,  Muller's 
tubercle,  into  the  dorsal  wall  of  the  urethra  (Fig.  142).  Thus  an  area, 
roughly  bounded  by  the  openings  of  the  ureters  and  the  mesonephric 
(ejaculatory)  ducts,  is  mesodermal.  Besides  the  trigone  of  the  bladder 
the  area  includes  a proximal  segment  of  the  urethra  (Fig.  160  C).  In 
the  male,  this  stretch  corresponds  to  the  upper  portion  of  the  prostatic 
urethra;  in  the  female,  it  includes  much  of  the  shorter  definitive  urethra. 

The  narrowed  apex  of  the  bladder,  continuous  with  the  allantoic 
stalk  at  the  umbilicus,  is  known  as  the  urachus  (Fig.  157).  It  persists 
as  the  solid,  fibrous  middle  umbilical  ligament  (Fig.  199).  Contrary  to 


148 


THE  UROGENITAL  SYSTEM 


earlier  views,  the  allantois  contributes  nothing  to  the  bladder  or  urachus 
(Felix,  1912). 

The  transitional  epithelium  of  the  bladder  appears  at  10  weeks.  The  circular  and 
outer  longitudinal  layers  of  muscle  develop  at  the  end  of  the  second  month.  The  inner 
longitudinal  muscle  layer  is  found  at  10  weeks  and  the  sphincter  vesicse  in  fetuses  of  three 
months. 

Anomalies. — A conspicuous  malformation  is  that  of  a persistent  cloaca,  due  to  the 
failure  of  the  rectum  and  urogenital  sinus  to  separate.  The  bladder  sometimes  opens 
widely  onto  the  ventral  body  wall  and  is  everted  through  the  fissure;  a urogenital  aper- 
ture corresponding  to  the  upper  extent  of  the  primitive  cloacal  membrane  would  cause  this 
condition  (Fig.  139  C,  D).  At  times,  the  urachus  remains  a patent  tube,  opening  at  the 
umbilicus  as  a urinary  fistula.  Portions  of  its  epithelium  which  fail  to  degenerate  may  form 
cysts. 


Fig.  143. — Reconstruction  of  the  male  urethra  and  associated  parts,  from  a fetus  of  four 
months  (after  Broman).  X 13. 

Accessory  Genital  Glands. — The  prostate  gland  develops  in  both  sexes 
as  outgrowths  of  the  urethra,  both  above  and  below  the  entrance  of  the 
male  ducts  (Fig.  143).  Hence,  the  upper  portion,  at  least,  must  be 
mesodermal  in  origin.  The  tubules  arise  at  ten  weeks  in  five  distinct 
groups  and  total  an  average  number  of  63.  The  surrounding  mesenchyme 
differentiates  both  connective  tissue  and  smooth  muscle  fibers,  into  which 
the  anlages  of  the  prostate  grow.  In  the  female,  the  homologue  is  rudi- 
mentary; these  isolated  para-urethral  ducts  (of  Skene)  number  at  most 
three. 

The  bulbo-urethral  glands  (of  Cowper)  arise  in  male  embryos  of  nine 
weeks  as  solid,  paired  epithelial  buds  from  the  entoderm  of  the  urethra 
(Fig.  143).  The  buds  penetrate  through  the  mesenchyme  of  the  corpus 
cavernosum  urethree,  about  which  they  enlarge.  The  glands  branch,  and, 


THE  GENITAL  ORGANS 


149 


at  four  months,  the  epithelium  becomes  glandular.  The  vestibular  glands 
(of  Bartholin)  are  the  homologues  in  the  female  of  the  bulbo-urethral 
glands.  They  appear  at  the  same  age  as  the  male  glands,  grow  until 
after  puberty,  and  degenerate  after  the  climacterium. 

II.  THE  GENITAL  ORGANS 
A.  Indifferent  Stage 

The  Gonads. — In  origin  and  early  development,  the  ovary  and  testis 
are  identical.  The  urogenital  fold  (p.  135)  is  the  anlage  of  both  the  meso- 


Fig.  144. — Ventral  view  of  the  urogenital  folds  in  a human  embryo  of  9 mm.  (Kollmann). 

nephros  and  the  genital  gland  (Figs.  392  and  144).  At  first  two-layered, 
its  epithelium  in  embryos  of  5 mm.  thickens  over  the  ventro-median 
surface  of  the  fold,  becomes  rciany-layered,  and  bulges  into  the  coelom 
ventrally  to  produce  the  longitudinal  genital  fold  (Fig.  13 1).  The  genital 
fold  thus  lies  mesial  and  parallel  to  the  mesonephric  fold.  Large  pri- 
mordial germ  cells  are  found  in  the  entoderm  of  the  future  intestinal 
tract;  at  3.5  mm.,  these  migrate  into  the  dorsal  mesenteric  epithelium 
and  thence  into  the  epithelium  of  the  genital  fold.  It  is  undecided 
whether  or  not  the  definitive  germ  cells  of  the  genital  glands  are  descen- 


THE  UROCxEXITAL  SYSTEM 


150 


dants  of  such  elements.  At  10  to  12  mm.,  the  genital  anlage  shows  no 
distinctive  sexual  differentiation  (Fig.  145);  there  is  a superficial  eph/je/ia/ 
layer  and  an  inner  epithelial  mass  of  somewhat  open  structure. 

Owing  to  the  great  development  of  the  suprarenal  glands  and  meta- 
nephroi,  the  cranial  portions  of  the  urogenital  folds,  at  first  parallel  and 
close  together,  are  displaeed  laterally.  This  produces  a double  bend  in 
each  fold,  which,  in  20  mm.  embryos,  shows  a cranial  longitudinal  portion, 
a transverse  middle  portion  between  the  bends,  and  a longitudinal  caudal 
portion  (Fig.  160  A).  In  the  last-named  segment,  the  mesonephric  ducts 
course  to  the  urogenital  sinus,  and  here  the  right  and  left  folds  fuse,  pro- 
ducing the  genital  cord  (h^ig.  154).  As  the  genital  glands  increase  in 
size,  they  become  constricted  from  the  mesonephric  fold  by  lateral  and 


Fig.  145. — Transverse  section  through  the  mesonephros,  genital  gland  and  suprarenal  gland 
of  the  right  side;  from  a 12  mm.  human  embryo  (Prentissf.  X 165. 

mesial  grooves  until  the  originally  broad  base  of  the  genital  fold  is  con- 
verted into  a stalk  (Figs.  149  to  151).  This  mesenterial  attachment 
extends  lengthwise  and  forms  in  the  male  the  mesorchium , in  the  female 
the  mesovarium. 

The  Primitive  Genital  Ducts. — The  mesonephric  ducts,  with  the 
degeneration  of  the  mesonephroi,  become  the  male  genital  ducts;  their 
origin  and  early  history  have  been  deseribed  (pp.  137  and  139). 

Both  sexes  also  develop  a pair  of  female  ducts.  In  embryos  of  10  mm., 
these  Mullerian  ducts  arise  as  thickened  ventro-lateral  grooves  in  the  uro- 
genital epithelium,  near  the  eranial  ends  of  the  mesonephroi  (Fig.  146  A). 


THE  GENITAL  ORGANS 


151 


A 

Lateral  body 
M iillerian  groove 


Anlage  of  Mullerian  duct 


Mesentery 


Mesonephric  riibule 


Genital  gland 


Fig.  146. — Transverse  sections  through  the  anlage  of  the  right  Mullerian  duct  from  a 10  mm. 
human  embryo  (Prentiss).  X 250.  A,  Cranial  end  of  groove;  B,  three  sections  caudad. 


Inferior  vena  cava 


Genital  gland 
Colon 


Allantois 


Pulmonary  artery 


Esophagus 

Mesonephric  duct 
Mesonephros 

Umbilical  artery 


Aorta 


Pulmonary  trunk 


Diaphragm 
Ostium  abdominale 
M iillerian 


Fig.  147. — X’entral  dissection  of  an  18  mm.  pig  embryo,  to  show  the  growing  Mullerian  ducts 

(Prentiss).  X 7. 


152 


THE  UROGENITAL  SYSTEM 


Fig. 


Esophagus 

Genital  gland 
Mesonephric  dud 

Allantois 


Lung 


Mesonephros 


M ullerian  dud 


Colon 


Umbilical  artery 


148. — Ventral  dissection  of  a 24  mm.  pig  embryo,  showing  a later  stage  in  growth  of  the 
Mullerian  ducts  (Prentiss).  X 6. 


Fig.  149. — Section  through  the  left  testis  and  mesonephros  of  a 20  mm.  human  embryo 

(Prentiss).  X 250. 


THE  GENITAL  ORGANS 


153 


Caudally,  the  dorsal  and  ventral  lips  of  the  groove  close  and  form  a tube 
which  separates  from  the  epithelium  and  lies  beneath  it  (Fig.  146  B). 
Cranially,  the  tube  remains  open  as  the  funnel-shaped  ostium  abdominale 
of  the  Mullerian  duct.  The  solid  end  of  the  tube  grows  caudalward, 
beneath  the  epithelium  and  lateral  to  the  mesonephric,  or  male  ducts  (Figs. 
147  to  149).  Eventually,  by  way  of  the  genital  cord,  the  Mullerian 
ducts  reach  the  median  dorsal  wall  of  the  urogenital  sinus  and  open  into 
it  (Figs.  142  and  160  A).  In  the  lov/est  vertebrates,  the  Mullerian  duct 
arises  by  a longitudinal  splitting  of  the  mesonephric  duct. 

Embryos  not  longer  than  12  mm.  are  thus  characterized  by  the  pos- 
session of  indifferent  genital  glands  and  both  male  and  female  genital 
ducts.  There  is  as  yet  no  sexual  differentiation. 

B.  Internal  Sexual  Transformations 

Differentiation  of  the  Testis. — In  male  embryos  of  13  mm.,  the 
genital  glands  show  two  characters  which  mark  them  as  testes:  (i)  the 


Fig.  150. — Section  through  the  left  testis  of  a fetus  of  fourteen  weeks  (Prentiss).  X 44. 

occurrence  of  branched,  anastomosing  cords  of  cells,  the  testis  cords;  (2) 
the  occurrence  between  epithelium  and  testis  cords  of  a layer  of  tissue,  the 
anlage  of  the  tunica  albuginea  (Fig.  149).  According  to  Felix  (1912),  the 
testis  cords  of  man  are  developed  suddenly  from  the  loose,  inner  epithelial 
mass  by  a condensation  of  its  cells;  on  the  contrary,  xMlen  (1904)  holds 
that  in  the  pig  and  rabbit  they  grow  in  from  the  surface  epithelium  .The 
cords  converge  towards  the  mesorchium,  where  they  form  the  dense, 
epithelial  anlage  of  the  slenderer  rete  testis.  Two  or  three  layers  of  loosely 
arranged  cells  between  the  testis  cords  and  the  epithelium  constitute  the 
future  tunica  albuginea. 

The  testis  cords  soon  become  rounded  and  are  marked  off  by  connec- 
tive-tissue sheaths  from  the  intermediate  cords,  which  are  columns  of  undif- 


154 


THE  UROGENITAL  SYSTEM 


ferentiated  tissue  lying  between  them  (Fig.  150).  Toward  the  rete  testis, 
the  sheaths  of  the  testis  cords  unite  to  form  the  anlage  of  the  mediastinum 
testis.  The  testis  cords  are  composed  chiefly  of  indifferent  cells,  with  a few 
larger  germ  cells.  The  cells  gradually  arrange  themselves  radially  about 
the  inside  of  the  conneetive-tissue  sheath  as  a many-layered  epithelium; 
during  the  seventh  month,  a lumen  ajDpears  and  extends  toward  the  rete 
testis  to  meet  lumina  which  have  formed  there.  Thus  the  solid  cords  of 
both  are  converted  into  tubules.  The  distal  portions  of  the  testis  tubules 
anastomose  and  form  the  tiihnli  contorti.  Their  proximal  portions  remain 
straight,  as  the  tuhuli  recti.  The  rete  testis  beeomes  a network  of  small 
tubules  that  finally  unite  with  the  efferent  duetules. 

ddie  primordial  germ  cells  of  the  testis  eords  form  the  spermatogonia 
of  the  seminiferous  tubules,  and  from  these,  at  puberty,  are  probably 
developed  the  later  generations  of  spermatogonia,  although  some  claim 
that  the  early  germ  cells  all  disappear,  to  be  replaced  later  from  the 
indifferent  elements.  The  indifterent  cells  of  the  tubules  become  the 
sustcntacnlar  cells  (of  Sertoli)  of  the  adult  testis.  Certain  cells  of  the  inter- 
mediate cords,  epithelial  in  origin,  are  transformed  into  large,  pale  cells, 
which,  after  puberty,  are  numerous  in  the  interstitial  connective  tissue  and 
hence  are  designated  interstitial  cells.  The  intermediate  cords,  as  such, 
disappear,  I:)ut  the  connective-tissue  sheaths  of  the  tubules  unite  to  form 
septula  which  extend  from  the  mediastinum  testis  to  the  fibrous  tunica 
albuginea. 

Differentiation  of  the  Ovary. — The  primitive  ovary,  like  the  testis, 
consists  of  an  inner  epithelial  mass,  bounded  by  the  parent  peritoneal 
epithelium.  The  ovarian  characters  ajjpear  much  more  slowly  than  those 
of  the  testis.  In  fetuses  of  ten  to  eleven  weeks,  the  inner  ei^ithelial  mass, 
composed  of  indifferent  cells  and  primordial  germ  cells,  becomes  less  dense 
centrally  and  bulges  into  the  mesovarium  (Fig.  151).  There  may  be  dis- 
tinguished a dense,  outer  cortex  beneath  the  epithelium,  a clearer  medul- 
lary zone  containing  large  germ  cells,  and  a dense,  cellular  anlage  in  the 
mesovarium,  the  primitive  rete  ovarii,  which  is  the  homologue  of  the  rete 
testis.  Neither  epithelial  cords  nor  tunica  albuginea  are  developed  at  this 
stage,  as  in  the  testis. 

Later,  three  important  changes  take  place:  (i)  There  is  an  ingrowth 
of  connective  tissue  and  blood  vessels  from  the  hilus,  resulting  in  the  forma- 
tion of  mediastinum  and  septula.  (2)  Most  of  the  cells  derived  from  the 
inner  epithelial  mass  are  transformed  into  young  ova,  the  process  extend- 
ing from  the  rete  ovarii  peripherally  (Fig.  151).  (3)  In  fetuses  of  three  to 

five  months,  the  ovary  grows  rapidly,  owing  to  the  formation  of  a new 
peripheral  zone  of  cells,  derived  perhaps  in  part  from  the  peritoneal  epithe- 
lium. At  the  end  of  this  period  the  septula  line  the  epithelium  with  a 


Fig.  152. — Section  through  the  ovarian  cortex  of  a five-months'  fetus  (De  Lee). 


Blood  vessel 


Primordial  ova 


THE  GENITAL  ORGANS 


fibrous  sheath,  the  anlage  of  the  tunica  albuginea.  Hereafter,  such  folds 
of  the  epithelium  as  form  do  not  penetrate  beyond  the  tunica  albuginea. 


Fig.  151 


Cortex 


-Section  through  the  left  ovary  of  a three-months’  fetus  (Prentiss).  X 44- 


Primordial  egg' 

Germinal  epithelium 


Tunica  albuginea 


Primordial  ovum 


Pfliiger's  egg  tubes' 


Tubules  of  mesonephros 
(Paroophoron) 


Rete  ovai .. 
in  mesovarium 


Epithelium 


Primordial 
germ  cells 


Medulla 


Uterine  (Mul- 
lerian) tube 


and  all  cells  derived  from  this  source  subsequently  degenerate.  This  new 
peripheral  zone,  according  to  Felix,  is  always  a single  cellular  mass  in 


156 


THE  UROGENITAL  SYSTEM 


man,  cords,  or  ‘Pfliiger’s  tubes,’  never  growing  in  from  the  epithelium. 
Generally,  it  has  been  believed  that  the  primary  follicles  are  derived  from 
the  subdivision  of  such  cords. 

Coincident  with  the  origin  of  a new  zone  of  cells  at  the  periphery  of 
the  ovary,  goes  the  degeneration  of  young  ova  in  the  medulla.  Invading 
connective  tissue  separates  these  germ  cells  into  clusters,  or  cords,  which 
degenerate  and  leave  only  a stroma  of  fibrous  tissue  in  the  medulla.  Late 
in  fetal  life,  indifferent  cells,  by  surrounding  the  young  ova  of  the  cortex, 
produce  primordial  follicles  (Fig.  13  A)  whose  differentiation  into  vesicular 
follicles  is  described  in  an  earlier  chapter  (p.  20).  In  opposition  to  this 
classic  concept,  Allen  ( 1923)  and  others  contend  that  the  definitive  ova  do 
not  represent  grown  primordial  ones  but  that  they  are  new  cells  proliferated 
periodically  in  the  adult  from  the  germinal  (peritoneal)  epithelium. 

Anomalies. — Congenital  absence  or  duplication  of  the  testes  and  ovaries  is  very  rare. 
Fused  te.stes  and  lobed  ovaries  are  also  known. 

Teratomata. — These  peculiar  tumor-like  growths  occur  rather  frequently  in  the  ovarv, 
less  often  in  the  testis  and  other  regions.  The  simpler  types,  called  dermoid  cysts,  contain 
such  ectodermal  derivatives  as  skin,  hair,  nails,  teeth,  and  sebaceous  glands.  They  grade 
into  comple.Kes  consisting  of  organ-like  masses,  from  all  three  germ  layers,  intermingled 
without  order.  Misshapen  representatives  of  all  tissues  and  organs  may  be  present. 
Among  other  explanations  of  the  cause,  the  isolation  and  subsequent  faulty  development 
of  blastomeres  has  been  advanced. 

Transformation  of  the  Mesonephric  Tubules  and  Ducts. — In  both 
male  and  female  embryos  of  21  mm.,  the  mesonephros  has  degenerated 
until  only  twenty-six  tubules  at  most  persist,  and  these  are  separated  into 
a cranial  and  a caudal  group.  In  the  cranial  group  of  5 to  12  tubules,  the 
collecting  jiortions  have  broken  apart  from  the  secretory  portions.  The 
free  ends  of  these  collecting  tubules  project  against  that  part  of  the  inner 
epithelial  mass  which  gives  rise  to  the  rete  tubules  of  either  testis  or  ovary 
(Figs.  149  and  151).  The  cords  of  the  rete  develop  in  contact  with  the 
collecting  tubules  of  the  mesonephros  and  unite  with  them  in  fetuses  of 
10  weeks. 

In  the  male,  the  lumina  of  rete  and  collecting  tubules  become  con- 
tinuous and  the  cranial  collecting  group  is  transformed  into  the  ductuli 
efcrentcs  of  the  epididymis.  During  the  fifth  month  of  pregnancy  the 
efferent  ductules  coil  at  their  proximal  ends,  and,  when  surrounded  by 
connective  tissue,  they  are  known  as  lobuli  epididymidis.  The  lower 
group  of  collecting  tubules  persist  as  the  vesitigial  paradidymis  and 
diiciuli  abherautes  (Fig.  160  G).  The  efferent  ductules  convey  spermatozoa 
from  the  testis  tubules  into  the  mesonephric  duct,  which  thus  becomes 
the  male  genital  duct.  The  cranial  portion  of  the  mesonephric  duct 
coils  and  forms  the  ductus  epididymidis;  its  blind  cranial  end  persists  as 


THE  GENITAL  ORGANS 


157 


the  appendix  epididymidis.  The  caudal  portion  of  the  male  duct  remains 
straight,  and,  as  the  diictiis  deferens  and  ejaculatory  duct,  extends  from  the 
epididymis  to  the  urethra.  Near  its  opening  into  the  latter  it  dilates  to 
form  the  ampulla,  from  the  wall  of  which  is  evaginated  the  sacculated 
seminal  vesicle  in  fetuses  of  three  months  (Fig.  143). 

In  the  female,  the  rete  ovarii  is  always  vestigial,  yet  some  time  before 
birth  it  becomes  tubular  and  unites  with  the  cranial  persisting  group  of 
mesonephric  collecting  tubules  which  forms  a rudimentary  structure,  the 
epodphoron  (Fig.  160  B).  The  caudal  group  of  mesonephric  tubules 
constitutes  the  paroophoron.  Usually  the  greater  part  of  the  meso- 
nephric ducts  atrophy  in  the  female,  the  process  beginning  early  in  the 
third  month,  but  portions  persist  as  Gartner’s  ducts  of  the  epodphoron. 

Gartner’s  ducts  may  extend  as  vestigial  structures  from  the  epodphoron  to  the  lateral 
walls  of  the  vagina,  passing  through  the  broad  ligament  and  the  wall  of  the  uterus.  They 
open  into  the  vagina  close  to  the  free  border  of  the  hymen.  The  ducts  are  rarely 
present  throughout  their  entire  length  and  are  absent  in  two-thirds  to  three-quarters  of  the 
cases  examined. 

Transformation  of  the  Mullerian  Ducts. — The  Mullerian,  or  female 
ducts,  follow  the  course  of  the  mesonephric  ducts  (Fig.  148).  At  first 
lateral  in  position,  the  Mullerian  ducts  cross  the  mesonephric  ducts  and 
enter  the  genital  cord  median  to  them  (Fig.  160  A).  In  embryos  of  two 
months  their  caudal  ends  are  dorsal  to  the  urogenital  sinus  and  extend  as 
far  as  the  Mullerian  tubercle,  a projection  into  the  median  dorsal  wall  of 
the  primitive  urethra  formed  by  the  earlier  entrance  of  the  mesonephric 
ducts  (Fig.  142).  This  tubercle  marks  also  the  position  of  the  future 
hymen.  In  fetuses  of  ii  weeks  the  Mullerian  ducts  break  through  the 
wall  of  the  urethra  and  open  into  its  cavity.  Before  this  takes  place, 
their  caudal  ends,  which  are  pressed  close  together  between  the  meso- 
nephric ducts  in  the  genital  cord,  fuse,  and  in  both  male  and  female 
embryos  of  two  months  give  rise  to  the  single  anlage  of  the  uterus  and 
vagina  (Figs.  142  and  153  A).  The  paired  cranial  portions  of  the  Miil- 
lerian  ducts  become  the  uterine  tubes.  During  development,  the  ostial 
ends  of  the  uterine  tubes  undergo  a true  descensus  from  the  third  thoracic 
to  the  fourth  lumbar  vertebra. 

In  the  male,  these  parts  are  rudimentary.  Those  portions  of  the  Mul- 
lerian ducts  corresponding  to  the  uterine  tubes  and  uterus  begin  to  degen- 
erate at  the  beginning  of  the  third  month.  The  vaginal  segment  remains 
as  a pouch  on  the  dorsal  wall  of  the  urethra,  the  vagina  masculina,  or 
prostatic  utricle  (Fig.  143).  The  extreme  cranial  end  of  each  Mullerian 
duct  constitutes  a so-called  appendix  testis  (Fig.  160  C). 

The  Uterus  and  Vagina. — Since  the  Mullerian  ducts  develop  in  the 
urogenital  folds,  they  make  two  bends  in  their  course  (Fig.  153  A)  cor- 


158 


THE  UROGENITAL  SYSTEM 


responding  to  those  of  the  folds  (p.  150).  Each  duct  consists  of  a cranial 
longitudinal  ])ortion,  a middle  transverse  portion,  and  a caudal  longi- 
tudinal jiortion  which  is  fused  with  its  fellow  to  form  the  utero-vaginal 
anlagc.  At  the  angle  between  the  cranial  and  middle  segments  is  attached 
the  inguinal  Jold,  the  future  round  ligament  of  the  uterus  (Figs.  154  and 
155).  The  mesenchyme  condenses  about  the  utero-vaginal  anlage  and 
the  middle  transverse  portion  of  the  Mullerian  ducts,  forming  a thick, 
sharply  defined  layer,  from  which  is  differentiated  later  the  muscle  and 
connective  tissue  of  these  organs  (Fig.  153  A).  As  development  proceeds, 
the  cranial  wall  between  the  transverse  limbs  of  the  Mullerian  ducts 


A Uterine  tube  B Fundus  of  uterus 


Fi'j-  153- — Diagrams  illustrating  the  development  of  the  uterus  and  vagina  (Felix-Prentiss). 

bulges  outward,  so  that  its  original  cranial  concavity  becomes  convex 
(Fig.  153  B).  The  middle,  transverse  portions  of  the  ducts  are  thus 
taken  up  into  the  wall  of  the  uterus  to  form  its  JunJus,  while  the  narrow 
cervix  of  the  uterus  and  the  vagina  arise  from  the  original  utero-vaginal 
anlage.  A distinction  between  uterus  and  vagina  is  not  evident  until 
the  middle  of  the  fourth  month.  The  entrance  to  the  vagina  is  originally 
some  distance  above  the  outlet  of  the  urogenital  sinus  (Fig.  160  A). 
This  intervening  stretch  of  sinus  hereafter  elongates  relatively  little  and 
so  becomes  the  shallow  vestibule  into  which  both  urethra  and  vagina  open 
(Fig.  160  B). 

The  lower  limit  of  the  vagina  lies  at  the  level  of  Muller’s  tubercle, 
where  the  utero-vaginal  anlage  breaks  through  the  wall  of  the  urogenital 
sinus.  The  tubercle  is  compressed  into  a disc,  lined  internally  by  the 
vaginal  epithelium  and  externally  by  the  epithelium  of  the  urogenital 
sinus,  or  future  vestibule.  These  layers,  with  the  mesenchyme  between 
them,  constitute  the  hymen,  which  thus  guards  the  opening  into  the 
vagina  (Fig.  160  A,  B).  A cireular  aperture  in  the  hymen  is  for  a time 
closed  by  a knob  of  epithelial  cells,  but  later,  when  the  hymen  becomes 
funnel-shaped,  the  opening  is  compressed  laterally  to  form  a sagittal  slit. 


THE  GENITAL  ORGANS 


159 


Muller’s  tubercle  persists  in  the  male  as  the  colliculus  seminalis,  from  the 
summit  of  which  leads  off  the  prostatic  utricle. 


At  10  weeks,  the  serosa,  muscularis,  and  mucosa  are  indicated.  The  first  circular 
muscle  fibers  appear  during  the  fifth  month;  the  other  muscle  layers  develop  later.  The 
epithelium  of  the  uterine  tubes  and  corpus  remains  simple;  that  of  the  cervix  and  vagina 
becomes  stratified  at  nine  weeks.  The  tubular  glands  of  the  corpus  appear  about  the 
seventh  month.  The  uterus  shortens  greatly  soon  after  birth  and  does  not  fully  recoup 
this  loss  until  the  eleventh  year.  The  virginal  size  is  attained  by  a short  period  of  rapid 
grovdh,  chiefly  before  puberty.  The  vagina  is  for  a time  without  a lumen,  and  solid 
epithelium  fills  its  fornices.  The  vaginal  lum.en  reappears  in  fetuses  of  about  five  months 
through  degeneration  of  the  central  epithelial  cells. 

Anomalies. — Many  cases  of  abnormal  uterus  and  vagina  occur.  The  more  common 
anomalies  are:  (i)  Complete  duplication  of  the  uterus  and  vagina,  due  to  the  failure  of  the 
Mullerian  ducts  to  fuse.  (2)  Uterus  bicornis,  due  to  the  incomplete  fusion  of  the  ducts. 
Combined  with  these  defects,  the  lumen  of  the  uterus  and  vagina  may  fail,  partly  or  com- 
pletely, to  develop  and  the  vaginal  canal  may  not  open  to  the  exterior  (imperforate  hymen) . 
(3)  The  body  of  the  uterus  may  remain  flat  (uterus  planifundus;  Fig.  153  .1)  or  fail  to  grow 
to  normal  size  (uterus  fetalis  and  infantalis).  (4)  Congenital  absence  of  one  or  both 
uterine  tubes,  or  of  the  uterus  or  vagina,  rarely  occurs,  but  may  be  associated  with  herma- 
phroditism of  the  external  genitalia.  The  hymen  is  of  variable  shape. 


Inguinal 


Gians  of  phallus 


Diaphragmatic  liga- 
ment of  mesonephros 


U reter 


Rcetum 


Genital  swelling 


an  duct 
in  mesonephric  fold 


Fig.  154. — Ventral  dissection  of  the  urogenital  organs  in  a human  embryo  of  two  months 
(Prentiss).  The  right  suprarenal  gland  has  been  removed  to  show  the  metanephros. 


Ligaments  of  the  Internal  Genitalia. — Female. — The  ovary  is  pri- 
marily suspended  by  a short  mesentery,  termed  the  mesovarium  (Fig.  151). 
A further  support  is  furnished  by  the  terminal  portion  of  the  primitive 
genital  fold,  which  unites  the  caudal  end  of  the  ovary  first  to  the  genital 
cord  and  then  to  the  uterus  that  develops  in  it.  This  connection  becomes 
fibrous  and  is  known  as  the  proper  ligament  of  the  ovary  (Fig.  15  s).  With 
the  degeneration  of  the  mesonephric  system,  the  uterine  tube  lies  in  a fold, 
the  mesosalpinx  (Fig.  151). 


i6o 


THE  UROGENITAL  SYSTEM 


The  mutual  fusion  of  the  caudal  portions  of  the  urogenital  folds,  as 
the  genital  cord,  forms  a mesenchymal  shelf  bridging  in  the  coronal  plane 
between  the  two  lateral  body  walls  and  containing  the  uterus  in  its  center 
(Fig.  154).  It  persists  as  the  sheet-like  broad  ligaments  of  the  uterus. 

In  embryos  of  14  mm.,  a band,  called  the  inguinal  fold,  joins  the  urogen- 
ital fold  to  the  inguinal  crest,  which  is  merely  a prominence  on  the  adjoining 
al)dominal  wall  (Fig.  154).  Within  the  inguinal  crest  is  differentiated  the 
chorda  giibcrnaculi,  which  later  becomes  fibrous.  The  abdominal  muscles 
develop  around  it  and  form  a tube,  the  inguinal  canal.  At  the  outer  end  of 
the  canal  the  external  obliciue  muscle  leaves  a foramen,  through  which  the 


Omr 

Ligamentum  ovarii 
Round  ligament  of  utcru 


Clitoris 


Labial  swelling 


Gians  clitoridis 


Suprarenal  gland 


Diaphragmatic 


Metanephros 
Pelvis  of  metanephros 


Uterine  tube 
Rectum 

Utero-vaginal  anlage 
Bladder 


Fig.  1 55. — Ventral  dissection  of  the  urogenital  organs  in  a female  fetus  of  nine  weeks  (Prentiss). 


chorda  connects  with  a second  cord  that  extends  to  the  genital  swelling 
and  is  hence  designated  the  ligamentum  labiale.  The  chorda  gubernaculi 
and  the  ligamentum  labiale  thus  form  a continuous  cable  from  the  labium 
majus  to  the  uterus,  which  in  the  meantime  has  been  developing  in  the 
fused  urogenital  folds;  the  two  together  constitute  the  round  ligament  of 
the  uterus  (F'ig.  155). 

Male. — The  primitive  mesentery  of  the  testis  is  the  mesorchium  (Figs. 
149  and  150).  It  is  represented  in  the  adult  as  the  fold  between  the 
epididymis  and  testis.  The  degenerating  cephalic  end  of  the  mesonephros 
for  a time  constitutes  the  so-called  diaphragmatic  ligament  of  the  meso- 
nephros (Figs.  154  and  155). 


THE  GEXITAL  ORGANS 


l6l 


The  ligamentiim  testis,  like  the  ligamentum  ovarii,  develops  in  the 
lower  end  of  the  genital  fold  and  extends  from  the  caudal  pole  of  the  testis 
to  the  mesonephric  fold  at  a point  adjacent  to  the  attachment  of  the  bridge- 
like inguinal  fold  (cf.  Fig.  155).  As  in  the  female,  the  inguinal  fold  con- 
nects with  the  chorda  guhernaculi  within  the  inguinal  crest,  and  this  in 
turn  is  continued  by  way  of  the  liganieutum  scroti  to  the  integument 
of  the  scrotum.  A cord  differentiates  in  the  mesonephric  fold  and  unites 
the  ligamentum  testis  to  the  chorda  gubernaculum.  Thus  there  is  formed 
a continuous  ligament,  the  gubernaculum  testis,  extending  from  the  caudal 
end  of  the  testis  through  the  inguinal  canal  to  the  scrotal  integument. 
The  gubernaculum  is  composed  of  the  ligamentum  testis,  a mesonephric 
cord,  the  chorda  gubernaculi,  and  the  ligamentum  scroti.  It  is  the 
homologue  of  the  ovarian  ligament  plus  the  round  ligament  of  the  uterus, 
between  which  the  uterus  intervenes  (Fig.  155). 

Descent  of  the  Testis  and  Ovary. — The  original  positions  of  the  testis 
and  ovary  change  during  development.  At  first  they  are  elongate  struc- 
tures, extending  in  the  abdominal  cavity  from  the  diaphragm  toward  the 
pelvis  (Fig.  144).  Since  their  caudal  ends  continue  to  grow  and  enlarge 
while  their  cranial  portions  atrophy,  there  is  a progressive,  wave-like  shift- 
ing of  the  glands  caudad.  Yet  an  actual  internal  descent  by  mass  move- 


Abdominal  cavity 


Abdominal  cavity 


Fig.  156. — Diagrams  illustrating  the  descent  of  the  testis,  p.v.,  Processus  vaginalis:  .v., 

obliterated  vaginal  sac. 


ment  does  not  occur.  When  the  process  of  growth  and  degeneration  is 
complete,  the  caudal  ends  of  the  testes  lie  at  the  boundary  line  between 
the  abdomen  and  pelvis,  whereas  the  ovaries  are  located  in  the  pelvis 
itself,  a position  which  they  retain.  Owing  to  the  rotation  of  the  o\mry 
about  its  middle  point  as  an  axis,  it  takes  up  a transverse  position.  The 
ovary  also  rotates  nearly  180°  about  the  Mullerian  duct  as  an  axis,  and 
thus  comes  to  lie  caudal  to  the  uterine  tube. 

In  addition  to  its  early  apparent  migration,  the  testes  normally  leave 
the  abdominal  cavity  and  descend  bodily  into  the  scrotum.  At  the  begin- 
ning of  the  third  month,  while  the  testes  are  still  fairly  high  in  the  abdomen, 
sac-like  pockets  appear  in  each  side  of  the  ventral  abdominal  wall.  These 
are  the  anlages  of  the  vaginal  sacs,  and  during  the  fourth,  fifth,  and  sixth 


i62 


THE  UROGENITAL  SYSTEM 


fetal  months  the  testes  lie  near  them  without  change  of  position.  Each 
processus  {saccns)  vaginalis  evaginates  over  the  pubis,  through  the  inguinal 
canal,  and  into  the  scrotum  (Fig.  156).  During  the  seventh  to  ninth 
months  the  testes  also  descend  rapidly  along  the  same  path  (Fig.  157). 
Although  the  factors  involved  are  not  sufficiently  understood,  it  is  clear 
that  the  gubernaculum  testis  plays  an  important  part.  From  the  caudal 
pole  of  each  testis  the  corresponding  gubernaculum  extends  through  the 
inguinal  canal  to  the  scrotal  wall.  During  the  seventh  month  the  guber- 
naculum not  only  ceases  growth  but  actually  shortens  one-half.  The 
resultant  relative  and  actual  shortening  serves  to  draw  the  testes  into  the 


Fig.  157. — Ventral  dissection  of  a full-term  fetus  to  demonstrate  the  relation  of  the 
descended  testis  to  the  processus  vaginalis  (partly  after  Kollmann  and  Corning).  On  the  left 
the  peritoneum  is  intact;  on  the  right  the  peritoneum  and  its  sacculation  are  opened  and  the 
testis  is  rotated  90°. 

scrotum  (Fig.  157),  where  they  usually  are  found  by  the  ninth  month,  or 
at  least  before  birth.  It  must  be  understood  that  the  testis  and  gubernacu- 
lum are  covered  by  the  peritoneum  before  the  descent  begins;  consequently 
the  testis  follows  the  gubernaculum  along  the  inguinal  canal  dorsal  to  the 
peritoneum,  and,  when  it  reaches  the  scrotum,  is  invaginated  into  the 
processus  vaginalis,  but  does  not  lie  within  the  cavity  of  the  coelomic 
extension  (Fig.  156).  The  gubernaculum  of  a newborn  is  but  one-fourth 
the  length  when  descensus  begins;  after  birth  it  atrophies  almost  completely. 

Within  a few  months  after  birth  the  narrow  canal  connecting  the 
processus  vaginalis  with  the  abdominal  cavity  becomes  solid  and  its 


Gubernaculum  testis 


Tunica  vaginalis 


THE  GENITAL  ORGANS 


163 


epithelium  is  resorbed.  The  vaginal  sac,  now  isolated,  represents  the 
tunica  vaginalis  of  the  testis.  Its  visceral  layer  is  closely  applied  to  the 
testis  and  its  parietal  layer  forms  the  lining  of  the  scrotal  sac.  The  ductus 
deferens,  and  the  spermatic  vessels  and  nerves,  are  carried  down  into  the 
scrotum  with  the  testis  and  epididymis.  They  are  surrounded  by  connec- 
tive tissue  and  constitute  the  spermatic  cord.  Owing  to  the  path  taken  by 
the  testis,  the  ductus  deferens  loops  over  the  ureter  in  the  abdomen 
(Fig.  160  C). 

In  the  female,  shallow  peritoneal  pockets,  frequently  persistent  as  the 
diverticula  of  Niick,  correspond  to  the  vaginal  sacs  of  the  male.  Rarely, 
a miOre  or  less  complete  descent  of  the  ovary  into  the  labium  majus  occurs. 
The  interposition  of  the  uterus  between  the  ovarian  and  round  ligaments 
is  responsible  for  the  normal  retention  of  the  ovaries  in  the  abdomen 
(Fig.  155). 

Anomalies. — At  times,  the  testes  remain  in  the  abdomen,  undescended,  a condition 
known  as  cryptorchism  and  associated  with  sterility  in  man.  In  some  mammals  (whale; 
elephant)  it  is  the  normal  condition.  When  the  inguinal  canals  of  man  remain  open,  con- 
ditions are  favorable  for  one  type  of  inguinal  hernia  of  the  intestine.  Open  inguinal 
canals,  with  a periodic  descent  during  the  breeding  season,  occur  normally  in  some  animals 
(rodents;  bats). 

C.  The  External  Genitalia 

Recent  investigation  (Spaulding,  1921)  proves  that  the  external  geni- 
talia exhibit  recognizable  sex  differences  almost  from  their  first  appear- 
ance. In  embryos  of  8 mm.,  a rounded  genital  tubercle  develops  in  the 
midline  of  the  ventral  body  wall,  between  the  umbilical  cord  and  tail 
(Fig.  144).  Its  caudal  slope  bears  the  shallow  urethral  groove  which  is 
separated  from  the  anal  pit  by  a transverse  ridge  (Figs.  158  A and  159  A) ; 
this  ridge  comprises  the  primitive  perineum.  The  margins  of  the  groove 
are  slightly  elevated  as  the  urethral  folds.  Embryos  of  about  15  mm.  show 
rupture  of  the  urethral  membrane  in  the  floor  of  the  groove,  and  the 
genital  tubercle  becomes  more  conical.  Sex  can  now  be  recognized  by 
the  length  of  the  urethral  groove  which  in  males  extends  from  the  base 
of  the  tubercle  nearly  to  its  apex  (Fig.  158  A,  B),  whereas  in  females  it 
is  shorter  and  terminates  some  distance  below  the  apex  (Fig.  159  A,  B)\ 
this  diagnostic  feature  prevails  until  the  definitive  modelling  begins. 

At  about  16  mm.  (seven  weeks)  the  genital  tubercle  has  elongated 
into  a somewhat  cylindrical  phallus,  bearing  at  its  tip  the  rounded  glans 
which  is  set  off  by  a constricted  neck  from  the  shaft-like  body  (Figs.  158  A 
and  159  A).  On  either  side  of  the  base  of  the  phallus,  and  separated 
from  it  by  a groove,  are  lateral,  rounded  ridges;  these  are  the  labio- 
scrotal  swellings,  possibly  represented  much  earlier  by  certain  indefinite 
elevations. 


164 


THE  UROGENITAL  SYSTEM 


Male.  —Embryos  of  ten  weeks  are  at  the  beginning  of  the  definitive 
stage.  In  the  male,  the  edges  of  the  urethral  groove  progressively  fold 
together  and  thus  transform  the  open  urogenital  sinus  into  the  tubular 
urethra  (Figs.  158  B,  C and  142).  The  fused  edges  constitute  the  raphe 
(Fig.  158  D).  The  scrotal  swellings  shift  caudad  to  their  final  position 
where  each  becomes  a half  of  the  scrotum,  separated  from  its  mate  by  the 
raphe  and  underlying  septum  scroti.  In  the  meantime,  the  shaft  of  the 


Urethral  fold  Gians  penis 


Fig.  158. — Stages  in  the  development  of  the  male  external  genitalia  (redrawn  after 
S]3aiilding).  .1,  Nearly  seven  weeks  (X  15);  B,  nearly  eight  weeks  (X  12):  C,  ten  weeks  (X  8); 
D,  twelve  weeks  ( X 8). 


polis  elongates,  and,  by  the  fourteenth  week,  the  urethra  has  closed  as 
far  as  the  glans.  The  urethra  is  then  continued  along  an  epithelial  plate 
which  represents  a solid  part  of  the  original  urethral  anlage  incompletely 
partitioning  the  glans;  by  splitting,  the  plate  is  first  converted  into  a 
trough  which  promptly  recloses  into  a tube  that  continues  the  urethra  to 
the  definitive  opening  at  the  tip  of  the  glans.  A cylindrical  collar  of 


THE  GENITAL  ORGANS 


165 


the  surface  epithelium,  incomplete  on  the  anal  side,  grows  deep  into  the 
end  of  the  primitive  glans.  By  the  disappearance  of  the  central  cells  of 
the  epithelial  downgrowth,  an  outer  cylindrical  mantle,  the  prepnciimi, 
or  foreskin,  is  formed  about  the  spheroidal  glans  penis  (cf.  Fig.  86). 
Where  the  epithelial  downgrowth  is  incomplete  the  glans  and  fore-skin 
remain  connected  by  the  frenulum  prepucii.  The  coropora  cavernosa 
penis  arise  as  paired  mesenchymal  columns.  The  corpus  cavernosuni 


Clans  clitoridis 
Urethral  groove 


Urethral  groove 
Urethral  fold 


Urethral  fold  Glans  clitoridis 

Labial  swelling  swelling 


Fig.  159. — Stages  in  the  development  of  the  female  external  genitalia  (redrawn  after 
Spaulding).  ,4,  Nearly  seven  weeks  (X  i8);  B,  nearly  eight  weeks  (X  15);  C,  ten  weeks 
(X  ii):  twelve  weeks  (X  8). 


urethra:  results  from  the  linking  of  similar,  unpaired  anlages,  one  in  the 
glans  the  other  in  the  shaft. 

Female. — Changes  in  the  female  are  less  profound,  yet  slower  (Fig. 
159).  The  phallus  lags  in  development  and  becomes  the  clitoris,  with 
its  homologous  glans  clitoridis  and  prepucium.  The  shorter  urethral 
groove  never  extends  onto  the  glans,  as  in  the  male.  It  remains  open  as 


i66 


THE  UROGENITAL  SYSTEM 


the  vestibule.  The  urethral  folds  which  flank  the  original  groove  con- 
stitute the  labia  niiiwra.  The  primitive  labio-scrbtal  swellings  grow 
caudad  and  fuse  in  front  of  the  anus  as  the  posterior  commissure  (embryos 
of  II  weeks),  while  the  original  lateral  portions  enlarge  into  the  labia 
majora;  these  parts  now  form  a horse-shoe  shaped  rim,  open  toward  the 
umbilicus.  The  mans  pubis,  which  arises  later,  appears  to  develop 
independently. 

Besides  the  sexual  difference  in  the  length  of  the  urethral  groove 
already  mentioned,  male  embryos  of  more  than  20  mm.  are  characterized 
by  a phallus  which  stands  at  right  angles  to  the  body,  whereas  in  the 
female  it  curves  downward. 


HOMOLOGIES  OF  INTERNAL  AND  EXTERNAL  GENITALIA 


Male 

Indifferent  stage 

Female 

Testi.s 

Rete  testis. 

Gonad. 

Ovary 

[Rete  ovarii]. 

Ligamentum  testis. 
Gubernaculum  testis. 

Primitive  ligaments. 

Ligamentum  ovarii  proprium. 
Lig.  ovarii  -|-  Lig.  teres. 

Ductuli  efferentes. 
Paradidymis. 

Mesonephric  collecting  tubules. 
Cranial  group. 

Caudal  group. 

Epobphoron. 

Paroophoron. 

Appendix  epididymidis 
Ductus  epididymidis. 
Ductus  deferens. 
Seminal  vesicle. 
Ejaculatory  duct. 

Mesonephric  duct. 

Gartner’s  duct. 

(1)  Appendix  testis. 

(2) 

(3)  Utriculus  prostaticus 
(Vagina  masculina). 

Mullerian  duct. 

(0  Uterine  tube. 

(2)  Uterus. 

(3)  Vagina. 

Colliculus  seminalis. 

Muller’s  tubercle. 

Hymen. 

(1)  Prostatic  and  mem- 
branous urethra. 

(2)  Cavernous  urethra. 

(3)  Prostate  gland. 

(4)  Bulbo-urethral  glands. 

Urogenital  sinus. 

(1)  Urethra  and  vestibule. 

(2) 

(3)  Para-urethral  ducts. 

(4)  Vestibular  glands. 

Penis. 

Gians  penis. 

Anal  surface  of  penis. 

Phallus. 

Gians. 

Lips  of  urethral  groove. 

Clitoris. 

Gians  clitoridis. 
Labia  minora. 

Scrotum. 

Labio-scrotal  swellings. 

Labia  majora. 
Posterior  commissure. 

THE  GENITAL  ORGANS 


167 


FEMALE 

Epididymis 


Fig.  160. — Diagrams  to  show  the  development  of  male  and  female  genital  organs  from  a 

common  type  (after  Thompson). 


i68 


THE  UROGENITAL  SYSTEM 


Anomalies.-  If  the  lips  of  the  slit-like  urogenital  opening  on  the  under  surface  of  the 
penis  fail  to  fuse,  hypospadias  results.  Rarely,  there  is  a similar  defect  on  the  upper  sur- 
face— epispadias;  it  is  usually  associated  with  vesico-abdominal  fissure. 

True  hermaphroditism  consists  in  the  presence  of  both  testis  and  ovary  in  the  same 
invividual.  It  is  of  rare  occurrence  in  birds  and  mammals,  is  not  uncommon  in  the  lower 
dertelirates,  and  is  the  normal  condition  in  many  invertebrates  (worms;  molluscs).  In 
man  there  are  five  authentic  cases  with  combined  ovotestis  and  four  cases  with  separate 
ovary  and  testis.  The  internal  genitalia  are  faultily  bisexual.  The  external  genitalia 
show  mixed  male  and  female  characteristics.  The  secondary  sexual  characters  (beard, 
mamm^,  voice,  etc.)  are  usually  intermediate,  tending  now  one  way,  now  the  other. 

False  hermaphroditism  is  characterized  by  the  presence  of  the  genital  glands  of  one 
sex  in  an  individual  whose  secondary  sexual  characters  and  external  or  internal  genitalia 
resemble  those  of  the  opposite  sex.  In  masculine  hermaphroditism,  an  individual  pos- 
sesses testes,  often  undescended,  but  the  external  genitals  (by  retarded  development)  and 
secondary  characters  are  like  tho.se  of  the  female.  In  feminine  hermaphroditism,  ovaries 
are  present,  and  sometimes  descended,  but  the  other  sexual  characters,  such  as  enlarged 
clitoris  or  fused  laldae,  simulate  the  male.  The  cause  of  hermaphroditism  is  unknown. 


CHAPTER  IX 


THE  VASCULAR  SYSTEM 
ORIGIN  OF  THE  BLOOD  VESSELS  AND  BLOOD  CELLS 

Both  the  primitive  blood  cells  and  blood  vessels  arise  from  a tissue 
termed  the  angioblast.  Its  germ-layer  origin  has  long  been  disputed,  but 
the  majority  of  recent  investigations  agree  on  the  mesoderm.  In  certain 
regions,  such  as  the  body  stalk  of  human  embryos,  any  other  interpretation 
is  precluded.  The  angioblast  consists  initially  of  isolated,  solid  cords  and 
masses  of  cells  which  appear  first  in  the  splanchnic  mesoderm  of  the  body 
stalk  and  yolk  sac  (Fig.  43).  These  strands  soon  hollow  out,  the  periph- 
eral cells  forming  the  flattened  endothelium  of  the  primitive  vessels, 
the  inner  cells,  bathed  by  a clear  fluid,  persisting  as  the  primitive  blood 
cells  (Fig.  326).  At  intervals,  clusters  of  the  latter  elements  adhere  to  the 
sides  of  the  vessels  and  constitute  the  temporary  blood  islands  (Figs.  44 
and  323). 

By  the  growth  and  union  of  the  isolated  spaces,  the  original  anlages 
are  converted  into  a vascular  plexus  which  is  present  on  the  yolk  sac,  body 
stalk,  and  chorion  of  human  embryos  of  i mm.  In  the  wall  of  the  yolk 
sac  this  network  comprises  the  area  vasculosa  which  later  envelops  the 
entire  sac. 

The  first  vessels  within  the  embryo  itself  appear  at  about  1.5  mm. 
Many  have  held  that  they  develop  as  continuations  of  the  extra-embryonic 
angioblast  which  progressively  invade  the  embryo,  but  it  is  now  agreed 
that  the  fundamental  origin  of  intra-embryonic  vessels  is  from  discrete 
local  anlages  like  those  on  the  yolk  sac.  Growth  by  sprouting,  rapidly 
extends  the  primitive  vascular  channels. 

HEMOPOIESIS 

Two  sharply  contrasted  views  are  held  as  to  the  mode  of  origin 
(hemopoiesis)  of  the  various  blood  elements.  According  to  the  monophy- 
letic  theory,  a common  mother  cell  gives  rise  to  all  types  of  blood  elements, 
both  red  and  white.  The  polyphylctic  theory,  on  the  contrary,  asserts 
that  the  erythroplastids  are  derived  from  one  mother  cell  while  the  several 
kinds  of  white  cells  trace  their  ancestry  to  one  or  more  distinct  stem  cells. 
The  total  evidence  favors  the  monophyletic  view. 

The  earliest  blood  cells  that  originate  from  the  angioblast  are  viewed 
by  some  as  the  parent  elements  from  wTich  all  later  blood  cells  are  derived. 

169 


170 


THE  VASCULAR  SYSTEM 


Although  it  is  recognized  that  various  organs  of  the  embryo  successively 
serve  as  blood-forming  centers,  they  are  interpreted  as  mere  depots  where 
the  primitive  angioblastic  cells  are  first  deposited  from  the  circulating 
blood  and  subsequently  proliferate.  On  the  contrary,  it  is  urged  by 
many  that  there  is  evidence  of  the  continued  new  formation  of  blood  cells 
from  the  mesenchyme  and  endothelium  of  the  embryo  and  from  the  con- 
nective tissue  of  the  adult.  This  is  the  more  popular  interpretation. 

The  primitive  blood  cells  multiply  rapidly  by  mitosis,  and  differen- 
tiate successively  in  the  following  locations;  (i)  yolk  sac;  (2)  mesen- 
chyme and  blood  vessels  of  the  embryo;  (3)  liver  and  spleen  (assisted  by 
lymphoid  organs  in  lymphocyte  production) ; (4)  bone  marrow.  There  is 
a certain  degree  of  overlap  in  the  activities  of  these  foci,  which,  one  by  one, 
give  up  blood  formation  until  the  red  marrow  alone  remains  as  the  per- 
manent source  of  all  types  of  blood  cells.  Yet  every  lymphoid  organ 
continues  the  production  of  lymphocytes  throughout  life,  and  in  certain 
diseases  the  spleen  assumes  again  its  full  hemopoietic  function. 

The  primitive  blood  cell  has  been  given  various  names,  such  as 
mcsameboid,  primary  lymphocyte,  and  hemoblast.  It  shows  a large,  vesicu- 


( j 

l!  /,  d 


Fig.  161. — Blood  cells  from  luiman  embryos  (Prentiss).  X 1160.  a.  Primitive  hemoblasts; 
h,  mcgaloblasts;  c,  d,  e,  normoblasts:  /,  erythrocytes,  (a-c,  12  mm.;  d-f,  20  mm.) 

lar  nucleus  surrounded  by  a small  amount  of  finely  granular  cytoplasm 
(Fig.  1 61,  a).  There  is  no  distinct  cell  membrane  and  the  cell  is  assumed 
to  be  ameboid.  From  such  parent  cells,  according  to  the  monophyletic 
view,  all  blood  elements  arise.  Specialization  proceeds  in  divergent  direc- 
tions; one  line  leads  to  the  red  corpuscles,  the  other  to  the  leucocyte  series. 

Origin  of  the  Erythrocyte. — The  red  blood  corpuscles,  arising  from  the 
hemoblast  type  of  cell,  are  first  formed  in  the  mesenchyme  and  blood  vessels 
of  the  e/mbryo  and  then  in  the  liver,  spleen,  and  bone  marrow.  Soon  after 


Fi(i.  162. — Human  blood  cells  (Todd).  X 1000.  i,  Erytliroplastid;  2,  normoblasts;  3, 
megaloblast  and  normoblast;  4,  blood  platelets,  one  lying  on  a I'ed  corpuscle;  5,  lymphocytes, 
large  and  small;  6,  7,  large  mononuclear  leucocytes,  polar  and  profile  views;  8,  neutrophilic 
leucocytes;  9,  eosinophilic  leucocytes;  10,  basophilic  leucocyte;  ii,  neutrophilic  myelocyte;  12, 
eosinophilic  megalocyte;  13,  basophilic  myelocyte. 


HEMOPOIESIS 


171 

birth,  the  red  marrow  is  the  only  normal  source  of  new  corpuscles.  In  each 
of  these  sites  the  manner  of  transformation  from  the  parent  hemoblast  is 
identical.  There  are  recognized  three  principle  stages : 

1.  Megaloblasts.  These  are  sometimes  called  erythroblasts  and  they 
have  also  been  termed  ichthyoid  blood  cells,  because  of  their  resemblance  to 
the  typical  red  blood  cell  of  fishes.  They  are  characterized  by  the  pres- 
ence of  hemoglobin  in  the  homogeneous  cytoplasm,  which  is  thus  colored 
red.  The  nuclei  are  ve.sicular,  with  granular  chromatin  (Figs.  16 1,  b 
and  162,  3).  There  is  a definite  cell  membrane.  For  the  first  six  weeks  of 
development  (12  mm.)  the  megaloblast  is  the  only  red  blood  cell  found, 
and,  like  its  progenitor,  multiplies  in  the  circulating  blood.  After  the  third 
month  it  practically  disappears  from  the  blood  stream. 

2.  Normoblasts,  also  termed  sauroid  blood  cells  because  they  resemble 

the  red  blood  cells  of  adult  reptiles  and  birds,  are  first  transformed  in  the 
liver  from  the  megaloblasts,  and  are  predomin- 
ant in  embryos  of  two  months.  They  are 
distinguished  by  their  small,  round  nuclei  with 
dense  chromatin  which  stains  so  heavily  that 
little  or  no  structure  can  be  seen  (Figs.  16 1,  c, 
d,  e and  162,  2,  3).  The  cytoplasm  is  large  in 
amount  and  contains  more  hemoglobin  than 
before,  but  the  normoblast  may  still  undergo  corpuscles  in  the  cat 

mitosis.  The  final  state  is  often  listed  as  a (Howell),  a,  Successive  stages  in 
separate  stage,  the  erythrohlast.  Until  the  the  development  of  a normoblast; 
seventh  month  many  normoblasts  occur  in  ^^trusion  of  the  nucleus. 

the  circulating  blood. 

3.  Erythrocytes  (red  blood  corpuscles;  erythroplastids)  are  developed 
in  mammals  from  normoblasts  which  lose  their  nuclei.  The  way  in  wTich 
the  nucleus  disappears  is  disputed.  It  is  usually  said  to  be  extruded  as  a 
whole  or  in  fragments  (Fig.  163),  but  some  claim  it  is  absorbed  and  others 
state  that  the  cytoplasm  buds  away  from  the  nucleated  remnant. 

The  first  red  blood  corpuscles  are  spherical  and  are  formed  during 
the  second  month,  chiefly  in  the  liver.  During  the  third  month,  the 
enucleated  erythrocytes  predominate  (Fig.  161,  /).  Although  usually 
cup-like  in  preserved  material,  their  normal  adult  shape  is  that  of  a bicon- 
cave disc  about  7.5  m in  diameter.  Mature  erythroplastids  are  believed  to 
exist  not  more  than  a month. 

Origin  of  the  Leucocytes. — The  white  blood  cells  are  divided  into 
non-granular  and  granular  groups  (Fig.  162).  According  to  the  mono- 
phyletic  view,  it  is  held  that  both  types  are  derived  from  the  hemoblastic 
mother  cells. 


172 


THE  VASCULAR  SYSTEM 


I.  Non-granular  Leucocytes: 

1.  Lymphocytes  are  ordinarily  about  the  size  of  a red  corpuscle  but 
some  are  twice  as  large  ( Fig.  162,  5).  The  small  lymphocytes  are  supposed 
to  be  the  daughter  cells  of  large  lymphocytes;  the  large  are  the  small  ones 
grown  up.  Their  spherical  nucleus,  containing  numerous  small  masses  of 
chromatin,  stains  darkly  and  is  surrounded  by  a narrow  zone  of  clear, 
faintly  baso])hilic  cytoplasm.  Lymphocytes  constitute  from  22  to  25  per 
cent  of  the  leucocytes  in  adult  Idood  and  are  developed  both  in  the  marrow 
and  in  the  lymphoid  organs. 

2.  Large  monomiclcar  leucocytes  are  two  or  three  times  the  size  of  a red 
corpuscle  (Fig.  162,  5,  6).  They  possess  a clear  nucleus,  usually  indented, 
and  considerable  faintly  basophilic  cytoplasm.  The  large  mononuclears 
are  notably  phagocytic.  They  comprise  i to  3 per  cent  of  all  leucocytes 
and  are  developed  from  the  reticular  cells  of  lymph  glands,  and,  perhaps, 
from  endothelium  as  well. 


Fii..  164. — (lianl  cell  from  the  bone  marrow  of  a kitten,  showing  pseudopodia  extending  into  a 
blood  vessel  (F),  and  giving  rise  to  blood  platelets  {hp)  (Wright). 

II.  (L'aiiular  or  Polymorphonuclear  Leucocytes: 

The  generalized  blood-forming  cells  lodged  in  the  red  bone  marrow 
also  give  rise  to  myelocytes,  cells  with  round  or  crescentic  nuclei  and  granu- 
lar cytoplasm  (Fig.  162,  11-13).  By  undergoing  changes  in  the  form  and 
structure  of  their  nuclei,  and  in  the  size  and  staining  qualities  of  their 
cytoplasmic  granules,  the  myelocytes  transform  into  three  types  of 
granular  leucocytes; 

I.  Neutrophils  (70  to  72  per  cent  of  all  leucocytes;  Fig.  162,  8).  These 
have  a hnely  granular  cytoplasm  which  is  neutral  in  its  staining  reactions, 
coloring  by  the  interaction  of  both  acid  and  basic  stains.  In  development, 
their  nuclei  take  up  an  eccentric  position  and  become  crescentic,  horse-shoe 
shaped,  and,  in  the  older  stages,  lobate.  As  it  changes  in  form,  the  nucleus 
undergoes  pyknosis  and  stains  intensely. 


DEVELOPMENT  OF  THE  HEART 


173 


2.  Eosinophils  (2  to  4 per  cent  of  all  leucocytes;  Fig.  162,  9).  These 
are  characterized  by  coarse  cytoplasmic  granules  that  stain  intensely  with 
acid  dyes.  The  granules  apparently  differentiate  intracellularly  although 
they  have  been  interpreted  as  ingested  fragments  of  red  corpuscles  or 
muscle.  In  development  the  nucleus  becomes  bilobed. 

3.  Basophils  or  Mast  Leucocytes  (0.5  per  cent  of  all  leucocytes;  Fig. 
162,  10).  Their  nuclei  are  very  irregular  in  form  and  may  be  broken  down 
into  several  pieces  which  stain  intensely.  The  cytoplasmic  granules  are 
variable  in  number,  size,  and  form,  and  often  stain  so  heavily  with  basic 
dyes  as  to  obscure  the  nucleus.  Basophiles  are  often  regarded  as  degenera- 
ting granular  leucocytes,  but  this  view  is  not  entirely  convincing.  They 
are  distinct  from  the  ‘mast  cells’  of  the  tissues. 

Origin  of  the  Blood  Platelets. — In  the  bone  marrow  are  giant  cells 
known  as  megakaryocytes,  the  cytoplasm  of  which  shows  a darkly-staining, 
granular  endoplasm  and  a clear,  hyaline  ectoplasm  (Fig.  164).  They  origi- 
nate like  leucocytes  but  follow  a distinct  course  of  specialization.  It  has 
been  demonstrated  that  the  blood  platelets  represent  the  tips  of  cyto- 
plasmic processes  which  have  been  detached  from  the  giant  cells.  The 
central  granular  mass  of  the  platelets  represents  a portion  of  the  endoplasm. 
Genuine  giant  cells  and  blood  platelets  occur  only  in  mammals. 

DEVELOPMENT  OF  THE  HEART 

The  heart  of  the  lower  fishes  and  of  amphibians  develops  directly 
within  the  ventral  mesentery  of  the  fore-gut.  A tubular  cavity  first  ap- 
pears, about  which  the  cells  differentiate  into  endo-,  myo-,  and  epicardium. 

While  the  embryo  of  bony  fishes,  reptiles,  birds,  and  mammals  is 
still  flattened  on  the  surface  of  the  yolk,  paired  heart  anages  arise  which 
secondarily  grow  mesad  and  fuse.  These  anlages  are  first  composed  of 
aggregates  of  mesodermal  cells  which  appear  between  the  entoderm  and 
splanchnic  mesoderm;  such  paired  cellular  masses  are  present  in  the  Spee 
1.54  human  embryo  (Fig.  43).  They  soon  form  thin-walled  endothelial 
tubes  and  are  flanked  by  folds  of  splanchnic  mesoderm  that  bulge  laterally 
into  the  coelomic  cavity  (Figs.  165  A and  327).  As  the  embryo  grows 
away  from  the  yolk  and  the  fore-gut  is  formed,  the  entoderm  withdraws 
from  between  the  endothelial  tubes,  allowing  first  these  and  then  the 
mesodermal  folds  to  fuse  (Figs.  165  5,  C;  328  and  329). 

The  heart  is  now  a single  endothelial  tube,  lying  in  the  folds  of 
the  splanchnic  mesoderm  (Fig.  iii  A).  When  the  ventral  mesenterial 
attachment  presently  disappears,  the  heart  is  left  suspended  by  a tem- 
porary dorsal  mesocardiiim  in  a common  pericardial  chamber  (Fig.  165  C). 
The  endothelial  tube  forms  the  endocardium;  the  splanchnic  mesoderm 
later  gives  rise  to  the  epicardium  and  myocardium.  This  type  of  heart 
occursin  human  embryos  of  2 mm.  ( 5 or  6 somites.  Fig.  1 66)  and  shows  three 


174 


THE  VASCULAR  SYSTEM 


regions;  (i)  the  atrium,  which  receives  the  blood  from  the  primitive  veins; 
(2)  the  ventricle;  (3)  the  Imlh,  from  which  is  given  off  the  ventral  aorta. 

As  the  cardiac  tube  soon  grows  faster  than  the  pericardial  cavity  in 
which  it  lies,  it  bends  to  the  right,  thereby  throwing  the  bulbus  and  ven- 
tricle into  a U-shaped  loop  (Fig. 
167).  Four  regions  may  then  be 
distinguished ; ( i ) the  sinus 
Eel.  venosiis;  (2)  the  atrium,  also  thin- 
walled  and  lying  cranial  to  the 
Ent.  sinus;  (3)  the  ihick-waXled  ventri- 
cular limb,  ventrad  and  cau- 
dad  in  position;  (4)  the  bulbar 
limb,  cranial  to  the  ventricular 
limb  and  separated  from  it  by 


Fig.  165. — Diagrams  to  illustrate  the  origin  of  Fig.  166. — The  heart  of  a a mm. 

the  mammalian  heart.  Eel.,  Ectoderm;  End.,  endo-  human  embryo  in  ventral  view  (Mall), 
thelial  tubes;  Ent.,  entoderm;  Eg.,  fore-gut;  Msc.d.,  X 65.  The  open  tube  is  the  fore-gut. 
dorsal  mesocardium;  Ms.spl.,  splanchnic  mesoderm 
(epi-  and  myocardium). 


the  bulbo-ventricular  cleft.  Next,  the  bulbo-ventricular  loop  further 
shifts  its  position  until  its  base  is  directed  caudad  and  the  loop 
as  a whole  lies  ventrad  (Fig.  167  B).  At  the  same  time,  the  sinus 
venosus  is  brought  dorsal  to  the  atrium  and  the  two  assume  a posi- 
tion cephalad  of  the  bulbo-ventricular  loop  (Fig.  168  A).  These  changes 
thus  result  in  an  essential  reversal  of  the  primitive  positional  relations. 

The  right  fDortion  of  the  sinus  venosus  now  begins  to  grow  more 
rapidly  than  the  left,  this  being  due  to  a shift  in  the  flow  of  blood  from 


DEVELOPMENT  OF  THE  HEART 


175 


the  left  umbilical  vein  through  the  liver  to  its  right  side.  As  a result,  the 
enlarged  right  horn  of  the  sinus  opens  into  the  right  dorsal  wall  of  the 
atrium  through  a longitudinally  oval  foramen,  guarded  on  each  side  by 
valve-like  folds  (Fig.  170).  The  atrium  is  constricted  dorsally  by  the  gut, 
ventrad  by  the  bulbus.  It  therefore  can  enlarge  only  laterally,  and  in  so 
doing  forms  sacculations  which  become  the  future  right  and  left  atria 


A 


Bulbus 


Ventricle 


Atrium 


B 


B lilhus 


Atrium 

{behind  ventricle) 


Ventricle 


Fig.  167. — Ventral  views  of  the  early  human  heart  (His).  A,  2.15  mm.;  B,  3 mm. 


(Fig.  168  A,  B)  \ the  deep,  external  groove  between  the  atria  and  the 
bulbo-ventricular  part  of  the  heart  is  the  coronary  sulcus.  As  the  bulbo- 
ventricular  region  increases  in  size,  the  duplication  of  the  wall  between  the 
two  limbs  lags  in  development  and  finally  disappears  (Fig.  169),  leaving 

A B 


Atrium 

Bulbus 


Atrium 


B ulbus 

trial 

canal 


Intervent. 

sulcus 

Ventricle 


human  heart  (His).  .4,  4.3  mm.;  B,  10  mm. 


Fig.  168. — Ventral  views  of  the  early 


Ventricle 


the  proximal  portion  of  the  bulb  and  the  ventricular  limb  to  form  a single 
chamber,  the  primitive  ventricle.  In  an  embryo  of  5 mm.,  the  heart  is  thus 
composed  of  three  undivided  chambers:  (i)  the  sinus  venosus,  opening 
dorsad  into  the  right  dilatation  of  the  atrium;  (2)  the, bilaterally  dilated 
atrium,  communicating  by  the  single  transverse  atrial  canal  with  (3)  the 


176 


THE  VASCULAR  SYSTEM 


primitive  undivided  ventricle.  The  three-chambered  heart  is  persistent 
in  adult  fishes,  but  in  birds  and  mammals  a four-chambered  heart  is 
developed,  in  which  venous  blood  circulates  on  the  right  side  and  arterial 
l:)lood  on  the  left.  In  amphibians  and  reptiles,  transitional  types  occur. 

The  important  changes  next  to  be  considered,  leading  to  the  forma- 
tion of  the  four-chambered  heart,  are:  (i)  the  complete  partitioning  of 
the  atrium  and  ventricle,  each  into  right  and  left  side  chambers;  (2)  the 
incorporation  of  the  sinus  venosus  into  the  wall  of  the  right  atrium;  (3) 
the  longitudinal  division  of  the  bulb  and  its  distal  continuation,  the  trun- 
cus  arteriosus,  into  the  aorta  and  pulmonary  artery;  (4)  the  development 
of  the  semilunar  and  atrio-ventricular  valves.  The  heart  of  an  embryo 
■of  two  months  has  attained  its  general  structural  characteristics. 


Pulmonary  artery  Aorta 


Fig.  169. — The  incorporation  of  the  bulbus  into  the  right  ventricle  through  the  slower  develop- 
ment of  the  bulbo-vcntricular  fold. 


Origin  of  the 'Right  and  Left  Atria. — In  human  embryos  of  6 mm. 
there  develops  a thin,  sickle-shaped  membrane  from  the  mid-dorsal 
wall  of  the  atrium  (Figs.  170  and  171).  This  is  called  the  septum  pri- 
mnm  (I),  for  it  grows  toward  the  ventricle  as  a partition.  Simultaneously, 
endothelial  thickenings  appear  in  the  dorsal  and  ventral  walls  of  the 
canal  which  connects  atrium  with  ventricle  (Figs.  171  A,  B).  These 
endocardial  cushions  later  fuse,  and  divide  the.  single  atrial  canal  into 
right  and  left  atrio-ventricular  canals  (Fig.  176).  The  atrium  is  now 
partly  divided  into  right  and  left  atria,  which,  however,  still  communi- 
cate ventrad  through  the  interatrial  Joranien.  Next,  the  septum  I thins 
out  in  one  region,  and  a secondary  opening,  the  foramen  ovale,  appears  there 
(Figs.  170  and  171  B).  The  atria  are  then  connected  by  two  openings, 
the  oval  and  interatrial  foramina.  Soon,  the  ventral  and  caudal  edge 
of  septum  / fuses  with  the  endocardial  cushions,  which  have  in  turn  united 
with  each  other  (Figs.  170  and  1 7 1 C).  The  temporary  interatrial  foramen 
is  thus  obliterated,  but  the  foramen  ovale  persists  until  after  birth.  In 
■embryos  of  9 mm.,  the  septum  secundum  (11)  is  developed  from  the  dor- 


DEVELOPMENT  OF  THE  HEART 


177 


sal  and  cephalic  wall  of  the  atrium,  just  to  the  right  of  the  septum 
primum  (Fig.  170  (T).  It  is  important,  as  it  later  fuses  with  the  left  valve 
of  the  sinus  venosus,  whence  the  two  join  with  septum  I to  complete  the 
atrial  septum  of  the  late  fetal  and  adult  heart. 

Fate  of  the  Sinus  \'enosus  and  its  Valves. — The  opening  of  the  sinus 
venosus  into  the  dorsal  wall  of  the  right  atrium  is  guarded  by  a right  and 


C 


Fig.  170. — Horizontal  sections  through  the  chambers  of  the  human  heart  (adapted  by  Prentiss). 
X about  50.  A,  6 mm.;  B,  9 mm.;  C,  12  mm. 

left  valvular  fold  (Fig.  170).  Along  the  dorsal  and  cephalic  wall  of  the 
atrium  these  unite  to  form  the  so-called  septum  spiiriiim;  caudally,  the 
valves  flatten  out  on  the  floor  of  the  atrium.  In  embryos  of  six  to  eight 


178 


THE  VASCULAR  SYSTEM 


Fig.  171. — Lateral  dissections  of  the  human  heart,  viewed  from  the  left  side  (Prentiss). 
X about  38.  A,  6 mm.;  B,  9 mm.;  C,  12  mm.  Cor.  sin.,  Coronary  sinus;  D.  end.  c.,  dorsal 
endocardial  cushion;  For.  ov.,  foramen  ovale;  Int.  for.,  interatrial  foramen;  I.  v.  c.,  inferior  vena 
cava;  L.  air.,  left  atrium;  L.  va.  s.  v.,  left  valve  of  sinus  venosus;  L.  vent.,  left  ventricle;  Pid.  a., 
pulmonary  artery;  Piil.  v.,  pulmonary  vein;  Sept.  I,  Sept.  II,  septum  primum,  septum  secun- 
dum; Sup.  V.  c.,  superior  vena  cava;  V.  end.  c.,  ventral  endocardial  cushion. 


Semilunar  valve  of 
pulmonary  artery 


Septum  I 


R.  ventricle 


Foramen  ovale 
R.  valve  of  sinus  venosus 


. vena  cava 
Septum  II 


Inf.  vena  cava. 


Fig.  172. — Lateral  dissection  of  the  heart  of  a three-months’  fetus,  viewed  from  the  right 

side  (Prentiss).  X 12. 


DEVELOPMENT  OF  THE  HEART 


179 


weeks,  the  atria  increase  rapidly  in  size  and  the  lagging  right  horn  of  the 
sinus  venosus  is  taken  up  into  the  wall  of  the  right  atrium.  By  this 
absorption  the  superior  vena  cava  of  necessity  drains  directly  into  the 
cephalic  wall  of  the  atrium,  the  inferior  vena  cava  into  its  caudal  wall 
(Fig.  1 71  C).  The  transverse  portion  of  the  sinus  venosus,  persisting  as 
the  coronary  sinus  in  part,  likewise  opens  into  the  posterior  w^all  of  the 
atrium  (Figs.  173  and  174). 

The  right  valve  of  the  sinus  venosus  is  very  high  until  the  end  of  the 
third  month  and  nearly  divides  the  atrium  into  two  chambers  (Fig.  172), 


Crista  lerminalis 
Sept.  valve  of 

sinus  venosus 
Septum  I 


Inf.  vena  cava 
Valve  of  inf.  vena 


Valve  of  coronary 


Tricuspid 


«,it. 


valves 
of  pulmonary  artery 


Sup.  vena  cava  (opened) 


R.  ventricle 


Fig.  173. — Lateral  dissection  of  the  heart  of  a four-months’  fetus,  viewed  from  the  right  side 

(Prentiss).  X 7- 

but  later  it  diminishes  greatly  in  relative  size.  Its  cephalic  portion 
becomes  the  rudimentary  crista  terminalis  (Fig.  173);  the  remainder  is 
divided  by  a ridge  into  two  parts,  of  which  the  larger  cephalic  division 
persists  as  the  valve  of  the  inferior  vena  cava  (Eustachian  valve),  located 
at  the  right  of  the  opening  of  the  vein,  and  the  smaller  caudal  portion 
becomes  the  valve  of  the  coronary  sinus  (Thebesian  valve). 

The  left  valve  of  the  sinus  venosus  unites  with  the  septum  II,  and, 
after  the  second  month,  the  two  bound  an  oval  opening  whose  rim  is  the 
limbus  ovalis  (Figs.  173  to  175). 


i8o 


THE  VASCULAR  SYSTEM 


L.  ventricle 


Fig.  174  — Lateral  dissection  of  the  heart  of  a three-months’  fetus,  viewed  from  the  left  side 

(Prentiss).  X 8. 


Sup.  vena  cava 
Aorta 


Septum  II 


Pulmonary  trunk 


Coronary  sinus 


L.  atrium 


Bicuspid  valve 


Foramen  ovale 


Septum  I 

Inferior  vena  cava 


Fig.  I 75. — Lateral  dissections  of  the  human  heart,  viewed  from  the  left  side  (Prentiss).  A , 
two  months;  B,  four  months.  Bic.  va.,  Bicuspid  valve;  Cor.  sin.,  coronary  sinus;  For.  ov., 
foramen  ovale;  I.v.c.,  inferior  vena  cava;  L.  atr.  vent,  c.,  left  atrio-ventricular  canal;  L.  vent., 
left  ventricle;  Pul.  a.,  jiulmonary  artery;  Sept.  I,  Sept.  II,  septum  primum  and  septum 
secundum. 


DEVELOPMENT  OF  THE  HEART 


l8l 


Closure  of  the  Foramen  Ovale. — -The  growth  of  the  primitive  atrial 
septa  proceeds  in  such  a manner  that  the  free  edge  of  septum  II  overlaps 
the  foramen  ovale  in  septum  I (Figs.  171  C,  174  and  175).  During  fetal 
life  the  left  atrium  receives  little  blood  from  the  lungs,  so  that  the  pressure 
is  much  greater  in  the  right  atrium.  As  a result,  the  septum  I is  pushed 
to  the  left  and  the  blood  flows  from  the  right  into  the  left  atrium  through 
the  foramen  ovale.  After  birth,  the  left  atrium  receives  from  the  expand- 
ing lungs  as  much  blood  as  the  right  atrium,  hence  the  septum  I is  pressed 
against  the  limbus  of  the  previously  fused  septum  II  and  left  sinus  valve, 
and  unites  with  it.  The  depression  formed  by  the  thinner  walled  septum 
I is  the  fossa  avails. 

The  Pulmonary  Veins. — In  embryos  of  about  6 mm.,  a single  vein 
drains  into  the  caudal  wall  of  the  left  atrium  at  the  left  of  the  septum  I 
{ Fig.  I 7 1 C) . This  vessel  bifurcates 
into  right  and  left  pulmonary  veins 
which  in  turn  divide  so  that  two 
branches  extend  to  each  lung.  fVs 
the  atrium  grows,  these  pulmonary 
vessels  are  progressively  taken  up 
into  the  atrial  wall.  As  a result, 
at  first  two,  then  four  pulmonary 
veins  open  into  the  left  atrium. 

Origin  of  the  Aorta  and  Pul- 
monary Artery. — In  embryos  of  5 
mm.  there  arise  in  the  aortic  bulb 
(including  its  distal  truncus  arteriosus)  longitudinal  thickenings,  four 
in  the  distal  half,  two  in  the  proximal  half.  Of  the  four  distal  thicken- 
ings (Fig.  176),  two,  which  may  be  designated  a and  c,  are  larger 
than  the  other  thickenings,  b and  d.  Thickenings  a and  c,  which 
distally  occupy  left  and  right  positions  in  the  bulb,  meet,  fuse,  and 
divide  the  bulb  into  a dorsally  placed  aorta  and  ventrally  placed 
pulmonary  trunk  (Fig.  177).  Traced  proximally,  they  pursue  a clockwise, 
spiral  course,  a shifting  from  left  to  ventral,  and  c from  right  to  dorsal, 
both  becoming  continuous  with  the  proximal  swellings.  Thickenings 
b and  d are  also  prominent  at  one  point  proximally;  Avhen  the  bulb  in  this 
region  is  divided  by  ingrowing  connective  tissue  into  the  aorta  and  pul- 
monary artery,  the  aorta  contains  the  whole  of  the  thickenings  b and  half 
of  a and  c,  while  the  pulmonary  trunk  contains  the  whole  of  d and  half 
of  a and  c (Fig.  176).  Distally,  the  three  thickenings  now  present  in 
each  vessel  disappear,  but  proximally  they  enlarge,  hollow  out  on  their 
distal  surfaces  and  eventually  form  the  thin-walled  5c?n?7«»ar  ra/t'C5  (Figs. 
172  and  176).  The  anlages  of  these  valves  are  prominent  in  embryos  of 


torta 


Piihnonary  artery 


Fig.  176. — Scheme  showing  the  division  of 
the  bulbus  and  its  thickenings  into  aorta  and 
pulmonary  artery  with  their  valves. 


i82 


THE  \'ASCULAR  SYSTEM 


six  to  seven  weeks  as  ])lump  swellings  projecting  into  the  lumina  of  the 
aorta  and  pulmonary  artery. 

The  two  proximal  bulbar  swellings,  continuous  with  a and  c,  fuse  and 
extend  the  spiral  division  of  the  bulb  toward  the  interventricular  septum 

A 


A oria 

Arnmy  in  pulmonary  artery 
Proximal  bulbar  sc  plum 


Interventricular 

foramen 

P atrio-ventricular 
foramen 


R.  ventricle. 


Pulmonary  artery 
Base  of  aorta 


Arrow  in  aorta 


L.  atrio-ventricular 
foramen 


I ntervcntricular 
septum 


Fig.  177. — Ventral  views  of  the  human  heart,  showing  the  division  of  the  bulbus  and  ventricle 

(Kollman).  A,  5 mm.;  B,  7.5  mm. 


in  such  a way  that  the  base  of  the  pulmonary  trunk,  now  ventrad  and  to 
the  right,  opens  into  the  right  ventricle,  while  the  base  of  the  aorta,  now 
lying  to  the  left  and  dorsad,  opens  into  the  left  ventricle  (Fig.  1 77  i?). 


DEVELOPMENT  OF  THE  HEART 


183 


Origin  of  the  Right  and  Left  Ventricles. — Coincident  with  the 
division  of  the  aortic  bulb  there  appears  at  the  base  of  the  primitive 
ventricular  cavity  a sagittally  placed  elevation,  the  interventricular  septum 
(Fig.  170  5).  It  grows  toward  the  endocardial  cushions,  and  temporarily 
forms  an  incomplete  partition  between  the  right  and  left  ventricles  which 
still  communicate  through  the  persisting  interventricular  foramen  (Fig. 
177  B).  Corresponding  to  the  internal  attachment  of  the  septum,  there  is 
formed  externally  the  interventricular  sulcus  (Fig.  177  A);  this  marks  the 
external  line  of  separation  between  the  large  left  ventricle  and  the  smaller 
right  ventricle.  The  interventricular  foramen  in  embryos  of  seven  weeks 
is  bounded:  (i)  by  the  interventricular  septum;  (2)  by  the  proximal  bulbar 
septum;  and  (3)  by  the  dorsal  portion  of  the  fused  endocardial  cushions 
(Fig.  177).  Soon  these  structures  are  approximated  and  fuse,  thereby 
forming  the  septum  memhranaceum,  which  closes  the  interventricular  fora- 
men and  completes  the  partition. 

Loosely-arranged  muscle  bundles  compose  the  uniformly  spongy 
wall  of  the  early  ventricle  (Fig.  178  .d).  Soon  there  is  a condensation, 
especially  at  the  periphery.  As  a result,  the  tissue  next  the  surface 
becomes  compact,  whereas  the  muscular  cords  near  the  lumen  retain 


Fig.  178. — Diagrams  of  the  development  of  the  ventricular  wall  and  atrio-ventricular  valves. 

an  open  arrangement  for  a longer  period  (Fig.  178  B).  Some  cords  are 
attached  to  the  anlages  of  the  atrio-ventricular  valves.  These  latter  arise 
as  thickenings  of  the  endocardium  and  endocardial  cushions,  about  the 
atrio-ventricular  foramina  (Figs.  170  and  171).  Three  such  flaps  are 
formed  on  the  right,  two  on  the  left.  The  size  of  the  primitive  valvular 
cusps  is  presently  increased  by  an  undermining  process  whereby  the 
muscular  cords  beneath  become  less  numerous  and  wider  spaced  (Fig. 
1 7 8 .S) . Degeneration  ensues  both  in  the  muscle  tissue  of  the  valve  anlages 
and  in  that  of  the  subjacent  muscle  cords.  As  a result,  the  valve  cusps 
become  fibrous  and  connect  with  similarly  transformed  chordce  tendinece, 
which  in  turn  continue  into  the  unaffected  papillary  muscles.  Thus 
there  are  developed  the  three  cusps  of  the  tricuspid  valve  between  the  right 


84 


THE  VASCULAR  SYSTEM 


chambers  of  the  heart  (Fig.  173)  and  the  two  flaps  of  the  bicuspid  {mitral) 
valve  between  the  left  chambers  (Fig.  174).  The  irregular  muscle  bundles 
that  ])ersist  next  the  ventricular  cavities  constitute  the  trabecula:  carnce. 

Differentiation  of  the  Heart  Wall. — The  primitive  folds  of  splanchnic  mesoderm  form 
both  the  thick  myocardium,  with  its  specialized  type  of  muscle,  and  the  serous  epicardial 
coat.  The  myocardial  layers,  at  first  continuous  over  the  surface  of  the  heart,  become 
divided  by  connective  tissue  at  the  atrioventricular  canal,  leaving  a small  bridge  alone. 
'I'his  connecting  strand,  located  behind  the  posterior  endocardial  cushion,  is  the  atrio- 
vaitriadar  hiuidlc.  The  endothelial  lining  becomes  the  chief  constituent  of  the  endocar- 
dium. Originally  a simple  sac,  it  later  dips  between  the  trabeculae  and  wraps  about  the 
papillary  muscles. 


Fig.  179. — The  caudal  end  of  a chick  embryo  of  32  somites  (Evans).  The  sciatic  artery 
will  differentiate  from  the  primary  capillary  plexus  of  each  limb  Irud;  aortae  have  already  formed 
from  the  mesial  margins. 

Descent  of  the  Heart.—  At  first  the  heart  lies  far  cephalad  in  the  cervical  region,  but 
it  gradually  recedes  during  development  until  it  assumes  a permanent  position  in  the 
thorax.  This  migration  is  attested  in  the  adult  by  the  courses  of  the  recurrent  and  cardiac 
nerves.  After  the  diaphragm  reaches  its  final  location  (Fig.  1 19),  the  heart  rotates  so  that 
the  ventricles,  which  previously  were  ventral  to  the  atria,  now  become  caudal. 

Anomalies. — Dextrocardia  is  associated  with  a general  transposition  of  the  viscera 
(p.  1 1 8).  The  aorta  and  pulmonary  artery  may  also  be  transposed  in  the  absence  of  dex- 
trocardia. Rarely,  the  paired  anlages  form  a double  heart.  Of  the  complete  or  partial 
defects  of  the  septa,  most  common  is  a patent  foramen  ovale.  If  the  foramen  fails  to  close 
after  birth,  the  mixed  blood  produces  a purplish  hue  in  the  child  which  is  known  popularly 


THE  PRIMITIVE  VASCULAR  SYSTEM 


185 

as  a ‘blue  baby.’  This  condition  may  be  persistent  in  adult  life.  Incomplete  closure 
occurs  in  about  one  in  four  cases,  but  actual  mingling  of  the  blood  is  rare,  due  to  an  approxi- 
mation of  the  overlapping  septal  folds  during  atrial  contraction.  Valvular  anomalies 
occur ; those  of  the  semilunar  valves  result  from  an  atypical  division  of  the  bulbus. 

THE  PRIMITIVE  VASCULAR  SYSTEM 

The  vascular  system  of  all  higher  mammals  develops  precociously. 
This  is  due  to  the  absence  of  nutritive  yolk,  and  the  consequent  need  of 
vessels  that  will  extract  nourishment  and  oxygen  from  the  maternal  circula- 
tion and  distribute  them  to  the  tissues  of  the  embryo. 

Delicate  injections  show  that  capillary  plexuses  precede  the  for- 
mation of  definite  arterial  and  venous  trunks  (Fig.  179).  Only  by  the 
selection,  enlargement,  and  differentiation  of  appropriate  paths  do  the 
definitive  vessels  arise,  whereas  those  capillaries  from  which  the  flow  has 
been  diverted,  atrophy.  Both  inheritance  and  the  hydrodynamic  factors 


Dorsal  intersegmental  arteries  Descending  aortce 


Fig.  180. — Diagram,  in  lateral  view,  of  the  blood  vessels  in  human  embryos  of  1.5  to  2 mm. 

(Felix-Prentiss). 


incident  to  the  blood  flow  participate  in  the  selection  of  channels  from  the 
capillary  bed. 

The  first  paired  vessels  of  human  embryos  are  formed  as  longitudinal 
anastomoses  of  capillary  networks  that  originate  first  in  the  angioblast  of 
the  yolk  sac  and  chorion  (p.  169).  In  the  Eternod  embryo  of  1.3  mm.,  in 
which  the  somites  are  still  undeveloped,  such  paired  vessels  are  already 
formed  (cf.  Fig.  iSo).  The  umbilical  veins  emerge  from  the  chorion, 
fuse  in  the  body  stalk,  then,  separating  again,  course  in  the  somatopleure 
to  the  paired,  tubular  heart  anlages.  From  the  heart  tubes,  paired  ves- 
sels, the  ventral  aorta:,  extend  cephalad,  then  bend  upward  around  the  first 
aortic  arches  and  continue  caudad  as  the  descending  aorta: . The  latter 
give  off  the  umbilical  arteries  which  bend  sharply  ventrad  into  the  body 
stalk  and  branch  in  the  wall  of  the  chorion.  The  chorionic  circulation  is 
thus  the  first  to  be  established. 


i86 


THE  VASCULAR  SYSTEM 


In  embryos  2 to  2.5  mm.  long  (5  to  8 somites),  the  heart  has  become 
a single  tube  (Fig.  181).  From  the  yolk  sac,  numerous  veins  converge 
cephalad  and  form  a pair  of  vitelline  veins.  These  join  the  umbilical  veins, 
whereupon  the  combined  vitcllo-iimhilical  trunks  traverse  the  septum 


Umbilical  arteries 


Dorsal  iiilersegmenlal  arteries  Precardinal  veins 

Descending  aortcE 


Body  stalk 

Umbilical  vein 

Vitelline  arteries 


Aortic  arch  1 

Heart 

V itdlo-umhilical  trunk 
Vitelline  veins 


Volk  sac 


Fig.  1 81. — Diagram,  in  lateral  view,  of  the  blood  vessels  in  human  embryos  of  2 to  2.5  mm. 

(Felix-Prentiss). 


transversum  and  open  into  the  sinus  venosus.  The  cranial  portions  of  the 
descending  aortas  give  off  several  pairs  of  dorsal  intersegmental  arteries, 
the  caudal  portions  a ventral  series  of  vitelline  arteries  to  the  yolk  sac. 

Postcardinal  veins  Precardinal  veins 


Fig.  182. — Diagram,  in  lateral  view,  of  the  blood  vessels  in  a human  embryo  of  2,6  mm. 

(Felix-Prentiss). 

The  umbilical  arteries  now  take  their  origin  from  a plexus  of  ventral 
vessels,  in  series  with  the  vitelline  arteries.  At  this  stage,  the  vitelline 
circulation  of  the  yolk  sac  is  established. 


Fig. 


183.- 


Ventride 


Liver  ■ 


R.  vitelline  vein 


Aortic  arches  T-4 
Ah  in  nt 

V itello-iimhilical  vein 
L.  umbilical  vein 


Ventral  reconstruction  of  the  blood  vessels  in  a 3,2  mm.  human  embryo  fHis). 


Allantois 


Olfactory  pit 
Manilihular  arch 

'Common  cardinal  vein 


Vitelline  vein 
U mbilical  vein 


Umbilical  vein 


■•Umbilical  artery 


Fig.  184. — Lateral  reconstruction  of  the  blood  vessels  in  a 4.2  mm.  human  embryo  (His). 
nix.,  Maxillary  process;  jv.,  precardinal  vein;  cv.,  postcardinal  vein;  ot.,  otocyst. 


■m 

■•I-hI 
” '■*1 

' 'I 


THE  PRIMITIVE  VASCULAR  SYSTEM 


187 


In  embryos  of  15  to  23  somites  (Fig.  182),  the  veins  of  the  embryo 
proper  develop  as  longitudinal  anastomoses  of  branches  from  the  segmental 
arteries.  The  paired  precardinal  (or  anterior  cardinal)  veins  of  the  head 
are  developed  first  (Fig.  18 1);  coursing  back  on  either  side  of  the  brain, 
they  join  the  vitello-umbilical  trunk.  In  embryos  of  23  somites,  the 
postcardinals  are  present  (Fig.  182).  They  lie  dorsal  to  the  nephrotomes, 
and,  running  cephalad,  join  the  anterior  cardinal  veins  to  form  the  common 
cardinal  veins.  Owing  to  the  later  enlargement  of  the  sinus  venosus,  the 


First  cervical  artery 
Pulmonary  artery 
Precardinal  vein 
Postcardinal  vein 

Subclavian  artery 
Cceliac  artery 


Dorsal  aorta 


Vertebral  artery 
Olocyst 


Primary  head  vein 


Bulbils  cordis 
Ophthalmic  artery 
Ant.  cerebral  artery 
Common  cardinal  vein 


Vitelline  artery 
{Superior  mesenteric) 

Caudal  artery 
Umbilical  arterv 


Inf.  mesenteric  artery 


Fig.  185. — Lateral  reconstruction  of  the  arteries  and  cardinal  veins  in  a 4.9  mm.  human 

embryo  (Ingalls-Prentiss).  X 20. 


proximal  portions  of  the  common  venous  trunks  are  taken  up  into  its  wall, 
and  thus  three  veins  open  into  each  horn  of  the  sinus  venosus:  (1)  the 
umbilical  veins  from  the  chorion;  (2)  the  vitelline  veins  from  the  yolk  sac; 
(3)  the  common  cardinal  veins  from  the  body  of  the  embryo.  The  descend- 
ing aorta:  fuse  below  the  level  of  the  seventh  intersegmental  arteries  and 
form  a single  dorsal  aorta  as  far  caudad  as  the  origin  of  the  umbilical 
arteries.  Of  the  numerous  vitelline  arteries,  one  pair  is  prominent;  its 
halves  unite  into  a single  vessel  which  courses  in  the  mesentery  and  later 
becomes  the  superior  mesenteric  artery.  By  the  enlargement  of  capillaries 


THE  VASCULAR  SYSTEM 


1 88 


connecting  the  ventral  and  dorsal  aorta},  a second  pair  of  aortic  arches  is 
formed  at  this  stage. 

In  embryos  4 to  5 mm.  in  length,  five  pairs  of  aortic  arches  are 
successively  developed:  the  first,  second,  third,  fourth,  and  sixth  (Figs. 
184  to  185).  An  additional  pair  of  transitory  vessels,  which  extend  from 
the  ventral  aorta  to  the  sixth  arch,  appear  later  in  embryos  of  7 mm.,  but 
soon  degenerate  (Fig.  186  B).  They  are  interpreted  as  being  the  fifth 


A 


Fig.  186. — Reconstructions  of  the  human  aortic  arches  and  pharyngeal  pouches  (Tandler;. 

A,  5 mm.;  B,  7 mm. 


pair  in  the  series.  From  each  dorsal,  or  descending  aorta  there  develop 
cranially  the  internal  carotid  arteries  (Fig.  184).  These  extend  toward 
the  optic  stalks  where  they  bend  first  dorsad  and  then  caudad,  and  connect 
finally  with  the  first  intersegmental  arteries  of  each  side  (Fig.  185).  The 
descending  aortae  are  now  fused  to  their  extreme  caudal  ends  and  the 
umbilical  arteries  thereby  originate  from  the  single  vessel.  Twenty-seven 
pairs  of  dorsal  intersegmental  arteries  are  jjresent ; from  the  seventh  cervical 
pair,  the  subclavian  arteries  of  the  upper  limbs  arise.  Of  the  ventral 
vitelline  vessels,  three  are  now  prominent : the  ccrliac  artery  in  the  stomach- 


DEVELOPMENT  OF  THE  ARTERIES 


189 


pancreas  region,  the  superior  mesenteric  in  the  small-intestine  region,  and 
the  inferior  mesenteric  of  the  large-intestine  region. 

The  embryonic  plan  of  primitive  vessels  is  altered  profoundly  in  later 
stages.  The  sections  that  follow  will  describe  these  changes  in  detail. 


DEVELOPMENT  OF  THE  ARTERIES 

Transformation  of  the  Aortic  Arches. — Both  the  ventral  and  descending 
aortae,  and  the  ancestral  aortic  arches  which  interconnect  them  (Fig.  186 
A),  are  early  transformed  into  more  appropriate  vessels.  In  embryos  of 
7 mm.,  the  first  and  second  pairs  of  aortic  arches  drop  out  (Figs.  186  B 
and  187),  but  the  subjacent  ventral  aortae  persist  as  the  external  carotid 
arteries;  similarly,  the  descending  aortse  at  this  level,  together  with  the 
third  aortic  arches,  become  the  internal  carotids.  The  continuations  of 
the  ventral  aorte  between  the  third  and  fourth  arches  remain  as  the 


External  carotid 


Innominate 

artery 


Right  sub- 
clavian artery 

Right  pul- 
monary artery 


Trunk  of  pul- 
monary artery 


Internal  carotia 
Commo7i  carotid 

Aortic  arch 

Ductus  arteriosus 

Vertebral  artery 

Subclavian  artery 

Left  pulmonary  artery 
Ventral  aorta 


Fig.  187. — Diagram,  in  ventral  view,  of  the  transformation  of  the  human  aortic  arches. 


common  carotid  stems,  whereas  the  corresponding  segments  of  the  descend- 
ing aortae  obliterate.  The  fourth  pair  of  aortic  arches  are  important ; the 
left  is  converted  into  the  permanent  aortic  arch:  on  the  right  side,  the 
fourth  arch  persists  with  the  descending  aorta  as  far  as  the  seventh 
intersegmental  artery  and  forms  the  first  part  of  the  right  subclavian  artery, 
which  is  thus  a more  complex  vessel  than  its  mate.  The  segment  of  the 
fourth  arch  proximal  to  the  right  common  carotid  becomes  the  innominate 
artery.  The  fifth  arches  of  amniotes  are  rudimentary  (p.  188).  On  the 
right  side,  the  distal  portion  of  the  sixth  arch  is  lost;  on  the  left,  it  persists 
as  the  ductus  arteriosus  and  its  lumen  is  obliterated  only  after  birth.  The 
proximal  portion  of  the  right  sixth  arch  forms  the  stem  of  the  right  pul- 


THE  VASCULAR  SYSTEM 


3 go 


monary  artery,  but  the  proximal  portion  of  the  left  arch  is  incorporated  in 
the  pulmonary  trunk.  Most  of  the  pulmonary  artery  arises  from  a post- 
branchial  plexus  whose  union  with  the  sixth  arch  is  acquired  secondarily 
(Huntington,  1919).  In  15  mm.  embryos,  the  primitive  bulbus  cordis  has 
been  divided  into  distinct  aortic  and  pulmonary  trunks  which  open 
respectively  into  the  left  and  right  ventricles. 

The  aortic  arches  of  the  embryo  are  of  especial  importance  comparatively.  Five 
arches  are  formed  in  connection  with  the  functional  gills  of  fishes.  In  adult  tailed  amphi- 
bia, three  or  four  arches,  and  in  some  reptiles,  two  arches,  are  represented  on  either  side. 
In  birds  the  right,  in  mammals  the  left  fourth  arch  persists  as  the  arch  of  the  aorta. 

The  different  courses  of  the  recurrent  laryngeal  nerves  are  easily  explained.  The  vagus 
early  gives  off  paired  branches  which  reach  the  larynx  by  passing  caudal  to  the  primitive 
fourth  aortic  arches.  When  the  latter,  through  growth  changes,  descend  into  the  chest, 
loops  of  both  nerves  are  carried  with  them.  Hence,  after  the  transformation  of  the  fourth 
arches,  the  left  recurrent  nerve  remains  looped  around  the  arch  of  the  aorta,  the  right 
around  the  right  subclavian  artery  (cf.  Fig.  187). 


Branches  of  the  Dorsal  Aorta. — ^From  each  primitive  aorta  arise 
dorsal,  lateral,  and  ventral  branches  in  three  paired  longitudinal  series 
(Fig.  188); 


Fig.  188. — Transverse  section  of  the  trunk,  illustrating  the  arrangement  of  the  segmental 

aortic  branches. 


I.  The  dorsal  branches  are  intersegmental  in  arrangement  and  develop 
small  dorsal  and  large  ventral  rami. 

From  the  dorsal  rami  are  given  off  neurdl  branches  which  bifurcate 
and  form  directly  the  dorsal  and  ventral  spinal  arteries.  The  vertebral 
arteries  arise  by  longitudinal,  postcostal  anastomoses  (Fig.  188)  of  the 
first  seven  pairs  of  dorsal  rami  (Fig.  189).  The  original  stems  of  the  first 
six  pairs  are  lost,  so  that  the  vertebrals  then  take  their  origin  from  the 
seventh  intersegmental  arteries  (Fig.  190).  In  embryos  of  9 mm.,  the 


DEVELOPMENT  OF  THE  ARTERIES 


I91 

vertebral  arteries  fuse  at  the  level  of  the  cerebellum  to  form  a single 
midventral  vessel,  the  basilar  artery;  since  the  internal  carotids  are 
recurved  cranially  at  5 mm.  (Fig.  185),  and  terminate  in  union  with  the 
first  intersegmental  arteries,  the  basilar  is  now  connected  cranially  with 
the  internal  carotids  and  caudad  with  the  definitive  vertebral  arteries. 

The  internal  carotids  (Fig.  185),  after  branching  off  the  ophthalmic  arteries,  give  rise 
cranially  to  the  anterior  cerebral  artery,  from  which  develop  later  the  middle  cerebral  and 
anterior  chorioidal  arteries;  all  of  these  supply  the  brain.  Caudalward  there  are  many  small 
branches  to  the  brain  wall  which  ultimately  form  a true  posterior  cerebral  artery. 


Fig.  189. — Origin  of  the  vertebral  and  subclavian  arteries  and  the  costo-cervical  trunk  in  a 
young  rabbit  embryo  (modified  after  Hochstetter).  Ill  AB.-IV  AB.,  Aortic  arches;  A.v.c.b., 
cephalic  portion  of  vertebral  artery;  C.d.  and  C.v.,  internal  and  external  carotid  arteries. 

The  ventral  rami  of  the  dorsal  intersegmental  arteries  become  promi- 
nent in  the  thoracic  and  lumbar  regions  and  persist  as  the  intercostal  and 
lumbar  arteries,  segmentally  arranged  in  the  adult.  Longitudinal,  pre- 
costal  anastomoses  (Fig.  188)  constitute  the  costo-cervical  and  thyro- 
cervical trunks  (Fig.  189).  The  subclavian  and  a portion  of  the  internal 
mammary  artery  are  derived  from  the  ventral  ramus  of  the  seventh  cervical 
segmental  artery  (Fig.  189).  The  remainder  of  the  internal  mammary, 


IQ2 


THE  VASCULAR  SYSTEM 


and  the  superior  and  injerior  epigastric  arteries,  are  formed  by  longitudinal 
ventral  anastomoses  (Fig.  1 88)  between  the  extremities  of  the  ventral  rami 


Fk;.  190. — Arterial  system  of  a human  embryo  of  lo  mm.  (His).  X i8.  Ic,  Internal  carotid 
artery;  P,  pulmonary  artery;  IV,  vertebral  artery;  111— VI,  persistent  aortic  arches. 


from  the  thoracic  and  lumbar  intersegmental  arteries,  beginning  with  the 
second  or  third  thoracic  (Fig.  191). 

2.  The  lateral  {visceral)  branches  of 
the  descending  aortae  are  not  segmentally 
arranged.  They  supply  structures  arising 
from  the  nephrotome region  (mesonephros, 
sex  glands,  metanephros,  and  suprarenal 
glands).  From  them  arise  the  renal, 
suprarenal,  inferior  phrenic,  and  internal 
spermatic  or  ovarian  arteries. 

Bremer  (1915)  derives  the  renal  ar- 
teries not  from  transformed  mesonephric 
vessels,  as  did  Broman  (1906),  but  from  a 
plexus  of  multiple  aortic  origin.  There 
cire  frequent  variations  in  the  selection  of 
])ermanent  channels. 

3.  The  ventral  {splanchnic)  branches 
are  imperfectly  segmental.  Primitively, 
they  form  the  paired  vitelline  arteries  to 
the  yolk  sac  (Figs.  180  to  182).  Coincident 
with  the  degeneration  of  the  yolk  sac,  the 
prolongations  of  the  ventral  vessels  to  its 
walls  disappear,  and  the  paired  arteries 

that  persist  and  pass  in  the  mesentery  to  the  gut  fuse  to  form 
unpaired  vessels.  From  these,  three  large  arteries  are  derived;  the  cceliac 


Fig.  191. — The  development  of  the 
internal  mammary  and  epigastric  ar- 
teries in  a human  embryo  of  13  mm. 
(Mall  in  McYIurrich). 


DEVELOPMENT  OF  THE  ARTERIES 


193 


artery,  the  superior  mesenteric,  and  the  inferior  mesenteric  (Figs.  185 
and  192). 

The  primitive  coeliac  axis  arises  opposite  the  seventh  intersegmental  artery.  It  then 
migrates  caudad  until  eventually  its  origin  is  opposite  the  twelfth  thoracic  segment  (Fig. 
192).  This  transference,  according  to  Evans,  is  due  to  the  unequal  growdh  of  the  dorsal 
and  ventral  walls  of  the  aorta.  Similarly,  the  superior  mesenteric  artery  is  displaced 
caudad  ten  segments,  the  inferior  mesenteric  artery  three  segments. 

The  umbilical  arteries  arise  in  embryos  of  2 to  2.5  mm.  from  the 
primitive  aortae  opposite  the  fourth  cervical  segment.  They  take  origin 
in  a plexus  of  ventral  vessels  of  the  vitelline  series  (Fig.  18 1),  and  are 
gradually  shifted  caudad  until  they  arise  from  the  dorsal  aorta  opposite 
the  twenty-third  segment  (fourth  lumbar).  In  5 mm.  embryos,  the 
umbilical  arteries  develop  secondary,  lateral  connections  with  the  aorta 
(Fig  192  A).  The  new  vessels  pass  lateral  to  the  mesonephric  ducts, 
and,  at  7 mm.,  the  primitive  ventral  stem-artery  has  disappeared  (Fig. 
192  B).  The  segment  of  this  new  trunk,  proximal  to  the  origin  of  the 
external  iliac  artery  which  soon  arises  from  it,  becomes  the  common  iliac. 


Fig.  192. — Reconstructions  of  the  human  aorta  and  its  branches  (Tandler-Prentiss).  -1, 

5 mm. ; B,  g mm. 

The  remainder  of  the  umbilical  trunk  constitutes  the  hypogastric  artery. 
When  the  placental  circulation  ceases  at  birth,  the  distal  portions  of  the 
hypogastric  arteries,  from  bladder  to  umbilicus,  atrophy,  forming  the 
lateral  umbilical  ligaments  of  adult  anatomy  (Fig.  199). 

The  middle  sacral  artery  is  the  direct  caudal  continuation  of  the  aorta. 
Its  dorsal  position  in  the  adult  is  the  result  of  secondary  growth  changes. 


13 


194 


THE  VASCULAR  SYSTEM 


Arteries  of  the  Extremities.-  It  is  assumed  that  in  man,  as  in  observed  birds  and  mam- 
mals, the  first  vessels  of  the  limb  buds  form  a capillary  jrlexus  (Fig.  i yg). 

U ppcr  Exirauily. — The  capillary  plexus  takes  origin  by  several  lateral  branches  from 
the  aorta.  In  human  emlrryos  of  5 mm.  but  one  connecting  vessel  remains,  and  this  arises 
secondarily  as  the  ventral  ramus  of  the  seventh  dorsal  intersegment  al  artery  (Fig.  iSSb  The 
portion  of  this  vessel  in  the  future  free  arm  is  plexiform  at  first,  ljut  later  becomes  a single 
axis  which  forms  successively  the  subclavian,  axillary,  brachial,  and  inicrosscous  arteries. 
Subseciuently,  the  median,  radial,  and  ulnar  arteries  develop. 

Lower  Extremity.^  In  embryos  of  7 mm.  a branch,  known  as  the  sciatic  artery,  is  given 
off  from  the  future  common  iliac.  It  is  the  cliief  arterial  stem  of  the  lower  extremity  and 
includes  the  definitive  popliteal  and  peroneal  arteries.  At  15.5  mm.  it  is  largely  superseded 
by  the  external  iliac  -and  femoral  arteric';,  of  which  the  latter  annexes  the  branches  of  the 
sciatic  distal  to  the  middle  of  the  thigh.  The  sciatic  artery  persists  proximally  as  the 
inferior  gluteal  artery. 


DEVELOPMENT  OF  THE  VEINS 

Three  systems  of  paired  veins  are  present  in  embryos  of  23  somites 
( b''ig.  182) : the  umbilical  veins  from  the  chorion;  the  vitelline  veins  from  the 
yolk  sac;  and  the  prccarditial  and  postcardinal  veins,  which  unite  in  the 
common  cardinal  veins,  from  the  body  of  the  embryo.  Thus,  three  veins 
open  into  the  right  horn  of  the  sinus  venosus,  and  three  into  the  left. 

Transformation  of  the  Vitelline  and  Umbilical  Veins. — Accompany- 
ing the  increase  in  size  of  the  primitive  liver  is  a mutual  intergrowth 
l)etween  the  hepatic  cords  and  the  endothelium  of  the  vitelline  A^eins. 
As  a result,  these  vessels  form  in  the  liver  a network  of  sinusoids  (Figs. 
183  and  193),  and  each  vein  is  thereby  divided  into  a distal  portion  which 
])asses  from  the  yolk  sac  to  the  liver,  and  into  a proximal  portion  which 
carries  blood  from  the  liver  sinusoids  to  the  .sinus  venosus.  Soon,  the 
proximal  part  of  the  left  vitelline  vein  is  largely  absorbed  into  the  hepatic 
sinusoids  and  shifts  its  blood  flow  to  the  right  horn  of  the  sinus  venosus. 
Moreover,  in  5 mm.  embryos,  the  parallel  vitelline  v’eins  communicate 
by  three  cross  anastomoses  (Figs.  194  and  195):  (i)  a cranial  transverse 
connection  in  the  liver,  ventral  to  the  duodenum;  (2)  a middle  one, 
dorsal  to  the  duodenum;  and  (3)  a caudal  one,  ventral  to  it.  There 
are  thus  formed  about  the  gut  a cranial  and  a caudal  venous  ring.  A 
new  vessel,  the  superior  mesenteric,  now  develops  in  the  mesentery  of  the 
intestinal  loop  and  joins  the  left  vitelline  vein  just  caudal  to  its  middle 
anastomosis  (Fig.  195).  Coincident  with  the  atrophy  of  the  yolk  sac, 
the  vitelline  veins  degenerate  caudal  to  the  junction  of  the  superior  mesen- 
teric vein.  The  persisting  trunk  between  the  latter  vessel  and  the  liver 
sinusoids  is  the  portal  vein,  and  thus  represents:  (i)  a portion  of  the  left 
vitelline  vein  in  the  left  limb  of  the  caudal  ring;  (2)  the  middle  transverse 
anastomosis  between  the  vitelline  veins;  (3)  that  segment  of  the  right 
vitelline  vein  which  forms  the  right  limb  of  the  cranial  ring. 


DEVELOPMENT  OF  THE  VEINS 


195 


Fig. 


.4 

Common  cardinal  vein 
Sinusoids  of  liver 


Ventricle 

Left  umbilical  vein 

T>-  It  ■.  11-  ■ 'll  i -i  ^ n ^ Left  vitelline  vein 

Kight  vitelline  vein  ■ ■ • 

193. — Ventral  reconstruction  of  the  blood  vessels  in  a 4.2  mm.  human  embryo  (His). 


Ductus  venosus 


Fig.  194. — Ventral  reconstruction  of  the  veins  of  the  liver  in  a 4.9  mm.  human  embrj'o  (Ingalls). 


THE  VASCULAR  SYSTEM 


196 


According  to  Mall,  the  intrahepatic  portion  of  the  right  vitelline  vein  persists  proxi- 
nially  as  the  right  ramus  of  the  hepatic  vein,  and  distally  as  the  ramus  arciiatus  of  the  portal 
vein.  The  intrahepatic  jjortion  of  the  left  vitelline  vein  drains  secondarily  into  the  right 
horn  of  the  sinus  venosus,  and  proximally  forms  later  the  left  hepatic  ramus.  Distally, 
where  it  is  connected  with  the  left  umbilical  vein,  it  becomes  the  ramus  angidaris  of  the 
portal  vein.  In  this  way  two  primitive  portal,  or  supplying  trunks,  and  two  hepatic,  or 
draining  trunks,  originate.  Later,  there  are  differentiated  first  four,  then  six,  such  apposed 
trunks  within  the  liver,  and  the  six  primary  lobes  supplied  and  drained  by  these  vessels 
may  be  recognized  in  the  adult. 

While  these  changes  have  been  progressing,  the  liver  tissue  grows 
laterad,  comes  in  contact  with  the  umbilical  veins,  and  taps  them  so  that 
their  blood  is  diverted  more  directly  to  the  heart  through  the  sinusoids  of 
the  liver  (Fig.  194).  As  the  channel  of  the  right  proximal  vitelline  is 
larger,  the  blood  from  the  left  umbilical  vein  flows  diagonal!}^  to  the  right 


Fig.  195. — The  origin  of  the  portal  vein  and  ductus  venosus  as  illustrated  by  a human  embryo 

of  7 mm.  (modified  after  Flis). 

horn  of  the  sinus  venosus.  When  all  the  umbilical  blood  enters  the  liver, 
as  in  embryos  of  5 to  6 mm.,  the  proximal  segments  of  the  umbilical  veins 
atrophy  (Fig.  195).  At  7 mm.  the  left  umbilical  is  large,  while  the  corre- 
sponding right  vein  has  degenerated  and  soon  disappears.  The  left  per- 
sists during  fetal  life,  shifts  to  the  midplane,  and  courses  in  the  free  edge 
of  the  falciform  ligament.  After  birth  its  lumen  is  obliterated,  and  from 
the  umbilicus  to  the  liver  it  constitutes  the  ligamentum  teres. 

In  the  liver,  the  portal  vein,  through  its  cranial  anastomosis  between 
the  primitive  vitelline  veins,  is  connected  with  the  left  umbilical  vein 
(Fig.  195).  As  the  right  lobe  of  the  liver  grows,  the  course  of  the  umbilical 
and  portal  blood  through  the  intrahepatic  portion  of  the  right  vitelline 
vein  becomes  circuitous,  and  hence  a new,  direct  channel  to  the  sinus 


DEVELOPMENT  OF  THE  VEINS 


197 


venosus  is  formed  through  the  hepatic  sinusoids.  This  is  the  ductus 
venosus  (Fig.  195),  which  is  obliterated  after  birth  and  forms  the  ligamen- 
tum  venosum  of  the  postnatal  liver. 

Transformation  of  the  Precardinal  Veins. — Each  precardinal  (anterior 
cardinal)  vein  consists  of  two  parts  (Fig.  185):  (i)  the  primary  head  vein, 
which  extends  into  the  unsegmented  head  proper  and  courses  ventro- 
lateral to  the  brain  wall;  (2)  the  true  precardinal,  located  laterad  in  the 
segmented  portion  of  the  head  and  neck  and  draining  into  the  common 
cardinal  vein. 


Aliddle  dural  plexus 


Posterior  dural  plexus 


Sinus  cavernosas 
A nierior  dural  plexus 


N.  hypoglossus 
Primary  head  vein 


Fig.  196. — Veins  of  the  head  in  a g mm.  human  embryo  (after  Mall).  X 9. 


The  primary  head  veins  have  three  pairs  of  tributary  plexuses  (Fig. 
196)  which  presently  extend  dorsad  over  the  brain.  From  this  primitive 
arrangement  the  various  veins  and  sinuses  of  the  brain  are  developed. 

The  true  precardinals  communicate  during  the  eighth  week  by  a 
transverse  venous  channel  which  carries  the  blood  from  the  left  side  of  the 
head  into  the  right  vein  (Fig.  197  D).  As  a result,  the  left  precardinal  soon 
loses  its  connection  with  the  common  cardinal  on  the  same  side  and 
degenerates  (E).  The  stump  of  the  left  common  cardinal  comprises  the 
inconstant  oblique  vein  of  the  left  atrium:  it  also  joins  with  the  transverse 
sinus  venosus  in  forming  the  coronary  sinus.  The  right  common  cardinal 
and  the  right  precardinal,  as  far  as  its  cross  anatomosis,  become  the 
superior  vena  cava.  The  anatomosis  itself  forms  the  left  innominate  vein, 


THE  VASCULAR  SYSTEM 


19S 


L.  innominaie 


J^rtcardinal 
Common  cardinal 

Ductus  venosus 
— Hepatic 
Posicardinal 

Subcardinal 

Mesonephros 


Subcardinal 

anastomosis 


Sex  gland 


Precardinal 


Postcardinal 


— Hepato-s  ubtardi- 
nal  union 
•Great  subcardinal 
\ anastomosis 


Subcardinal 

Ext.  iliac 
Hypogastric 


Supracardinal 


Suprarenal 


Sub-su  pracardi- 
nal  anastomosis 


Postcardinal 


Suh-siipracardinal 

anaslomosiSK 


Ext.  jugular 
Ini.  jugular 
R.  subclavian 

R.  iyxnominaie 
Su p.  vena  cava 


.4  zygos 

Hepatic  pari  of  inf. 

vena  cava. 


R.  suprarenal 


D 


L.  innominate 


Coronary  sinus 


Acc.  hemiazy- 
gos 


L.  spermatic 


L.  com.  iliac 


Ext.  iliac 
Hypogastric 

c ^ 

Fig.  IQ7.— Diagrams  to  illustrate  the  transformation  of  the  pre-,  post-,  sub-,  and  supra- 
cardinal  veins  (adapted  after  Huntington  and  McClure).  A,  6 mm.;  B,  10  mm.;  , 15  mm., 
D,  18  mm. 


DEVELOPMENT  OF  THE  VEINS 


199 


while  that  portion  of  the  right  precardinal  between  the  anastomosis  and 
the  right  subclavian  vein  is  known  as  the  right  innominate.  The  distal 
segments  of  the  precardinals  become  the  internal  jugular  veins  of  the  adult, 
whereas  the  external  jugular  and  subclavian  veins  are  vessels  which  develop 
somewhat  later  (C-E). 

Transformation  of  the  Post-,  Sub-,  and  Supracardinal  Veins. — The 

primitive  postcardinal  veins  course  cephalad  along  the  dorsal  sides  of  the 
mesonephroi  and  open  into  the  common  cardinals  (Fig.  197  *d).  Each 
receives  tributaries  from  the  posterior  extremities,  mesonephroi,  and 
body  wall.  Median  and  ventral  to  the  mesonephros  are  developed  the 
suhcardinal  veins,  which  connect  at  intervals  with  the  postcardinals 
through  the  mesonephric  sinusoids,  and  with  each  other  by  anastomoses 
ventral  to  the  aorta.  Thus,  all  the  blood  from  the  lower  body  is  in  early 
stages  drained  by  the  postcardinal  veins  alone.  Soon,  the  postcardinals 
are  divided  midwayinto  cranial  and  caudal  segments  (S).  Cranial  to  their 
interruption,  these  vessels  atrophy  (C).  The  caudal  portions  are  asso- 
ciated with  the  mesonephroi  and  persist  longer,  but  finally  disappear 
with  those  organs  {D,  E).  The  sole  permanent  remnants  of  the  post- 
cardinal system  are  small  contributions  to  the  azygos  and  sex  veins. 

Of  the  subcardinals,  only  the  middle  regions,  at  about  the  final  level 
of  the  kidneys,  are  retained.  Here  the  two  vessels  communicate  by  a 
broad  anastomosis,  and  here  each  is  similarty  connected  with  the  post- 
cardinal of  the  same  side  [B).  Below  this  level  the  subcardinals  presently 
disappear,  except  for  portions  which  supply  the  sex  glands  {C-E) . Above, 
the  left  drops  out,  its  lower  stump  alone  transforming  into  the  left  supra- 
renal vein;  the  corresponding  part  of  the  right  subcardinal  remains  as  the 
right  suprarenal  and  also  as  an  important  component  of  the  inferior  vena 
cava. 

In  the  meantime,  a new  pair  of  anastomosing  veins,  the  su pracardinals, 
make  their  appearance  (C).  They  lie  dorso-mesial  to  the  postcardinals, 
and,' in  a sense,  replace  them.  The  supracardinal  veins  originally  extend 
from  near  the  common  cardinals  to  the  union  of  the  primitive  iliac  vessels, 
but  they  soon  break  at  the  level  of  the  kidneys  (D).  The  cranial  halves 
midway  develop  a prominent  anastomosis  and  become  the  azygos  and 
hemiazygos  of  the  adult  (£).  Opposite  the  kidneys,  the  caudal  segments 
form  a permanent,  broad  union  with  the  right  subcardinal  and  a temporary 
one  with  the  left  (C,  D]  in  yellow).  The  right  main  supracardinal  chan- 
nel, with  the  annexed  right  subcardinal,  constitutes  the  lower  half  of  the 
inferior  vena  cava  {D,  E)  (Huntington  and  McClure,  1920). 

The  development  of  the  unpaired  inferior  vena  cava  begins  when  com- 
munication is  established  between  the  right  hepatic  and  right  subcardinal 
veins.  The  liver  on  the  right  side  becomes  attached  to  the  dorsal  body 


200 


THE  VASCULAR  SYSTEM 


wall,  and  from  its  point  of  union  a ridge,  the  caval  mesentery  (Fig.  112), 
extends  caudad.  Capillaries  from  the  subcardinal  vein  invade  the  mesen- 
tery, and,  growing  cranially,  meet  and  fuse  with  capillaries  extending 
caudad  from  the  liver  sinusoids.  Thus  is  formed  the  vein  of  the  caval 
mesentery  (.4,  B),  which  is  already  present  in  human  embryos  of  10  mm. 
44ie  blood  from  the  lower  trunk  and  leg  region  soon  becomes  drained  by 
the  complex  inferior  vena  cava,  which  is  composed  of  the  following  veins 
(E) : (1 ) the  common  hepatic  and  right  hepatic  veins  (primitive  right  vitel- 
line); (2)  the  connecting  vein  of  the  caval  mesentery;  (3)  an  inter-renal 
portion  of  the  right  subcardinal  vein  (and  its  adjoining  anastomoses  with 
the  right  post-  and  su])racardinal  and  the  left  subcardinal) ; (4)  the  right 
supracardinal  vein,  below  the  level  of  the  kidneys. 

The  permanent  kidneys  take  up  their  positions  opposite  the  great 
anastomosis  between  the  subcardinals,  and,  at  this  point,  the  renal  veins 
are  developed  [D,  E)\  the  longer  left  renal  vein  differs  from  the  right  in  that 
proximally  it  represents  a left  portion  of  the  anastomosis  itself.  A cephalic 
segment  of  the  left  subcardinal  vein  persists  as  the  left  suprarenal  vein, 
which  thus  opens  into  the  left  renal  instead  of  joining  the  inferior  vena  cava 
as  does  the  right  suprarenal  vein  of  similar  subcardinal  origin  (E).  The 
spermatic  or  ovarian  veins  contain  both  postcardinal  and  subcardinal 
components.  The  left  early  drains  into  the  left  caudal  border  of  the  great 
subcardinal  anastomosis,  which,  as  already  described,  contributes  to  the 
left  renal  vein;  the  right  opens  into  that  portion  of  the  right  subcardinal 
which  is  incorporated  into  the  inferior  vena  cava.  The  posterior  intercostal 
and  lumbar  veins  are  at  first  tributaries  of  the  postcardinals.  As  the  latter 
vessels  degenerate,  these  tributaries  connect  secondarily  with  the  replacing 
supracardinal  veins;  later,  they  of  necessity  drain  respectively  into  the 
azygos  veins  and  inferior  vena  cava.  The  history  of  the  common  iliacs 
is  similar,  the  stem  of  the  longer  left  representing  a caudal  anastomosis 
between  the  primitive  paired  supracardinals  (C-E). 

Veins  of  the  Extremities. — The  primitive  capillary  plexus  of  the  flattened  limb 
buds  gives  rise  to  a peripheral  border  vein  (Fig.  227).  In  the  upper  extremity,  its  ulnar  por- 
tion persists,  forming  at  different  points  the  subclavian,  axillary,  brachial,  and  basilic  veins. 
At  10  mm.  the  border  vein  opens  into  the  dorsal  wall  of  the  postcardinal,  but,  as  the  heart 
shifts  caudad,  it  Anally  drains  by  a ventral  connection  into  the  xjrecardinal,  or  internal  jugu- 
lar vein.  The  cephalic  vein  develops  secondarily  in  connection  with  the  ulnar  border  vein; 
later,  in  embryos  of  23  mm.,  it  anastomoses  with  the  external  jugular  and  finally  drains 
into  the  axillary  vein,  as  in  the  adult.  AVith  the  development  of  the  digits,  the  vv.  ccphalica 
and  basilica  become  distinct  (35  mm.),  but  later  are  again  connected  by  a plexus 
on  the  dorsum  of  the  hand. 

In  the  lower  extremity,  the  fibular  portion  of  the  primitive  border  vein  persists.  Later, 
the  V.  saphena  niagna  arises  separately  from  the  postcardinal,  gives  off  the  vv.  femoralis 
and  tibialis  posterior,  and  annexes  the  fibular  border  vein  at  the  level  of  the  knee.  Distal 
to  this  junction,  the  border  vein  persists  as  the  v.  tibialis  anterior,  and,  probably,  the 


FETAL  CIRCULATION 


201 


V.  saphena  parva;  proximally,  it  becomes  greatly  reduced,  forming  the  v.  glutea  inferior. 

Anomalies. — Anomalous  blood  vessels  are  of  common  occurrence.  They  may  be 
due:  (i)  to  the  choice  of  unusual  paths  in  the  primitive  vascular  plexuses;  (2)  to  the  per- 
sistence of  vessels  normally  obliterated;  (3)  to  the  disappearance  of  vessels  normally 
retained;  (4)  to  incomplete  development;  (5)  to  fusions  and  absorptions  of  parts  usually 
distinct. 


FETAL  CIRCULATION  AND  THE  CHANGES  AT  BIRTH 

During  fetal  life  oxygenated  placental  blood  enters  the  embryo 
by  way  of  the  large  umbilical  vein  and  is  conveyed  to  the  liver  where  it 
mingles  with  that  brought  in  by  the  portal  vein  (Fig.  198).  Thence  it 


Ductus  venosus 
Portal  vein 


Hypogastric  artery 
External  iliac  artery 


Umbilical  vein 
Inferior  vena  cava 

U mbilical 

U mbilical  lei 

Urach 


Pulmonary  artery 
Left  pulmoyiary  vein 
Left  atrium 


ventri.le 
Left  ventricle 


Superior  vena  cava 


Right  atriu 
Foramen 


I 

.4  orta 

Ductus  arteriosus 


Fig.  ig8. — Diagram  of  the  circulation  before  birth  (Heisler).  Arrows  point  out  the  course 
of  the  blood  current;  colors  show  the  older  conception  of  the  character  of  the  blood  carried  by 
various  vessels,  whereas  experimentation  indicates  a thorough  mixing  within  the  heart. 


flows  to  the  inferior  vena  cava  either  directly,  through  the  ductus  venosus, 
or  indirectly  through  the  liver  sinusoids  and  hepatic  vein.  The  impure 
blood  of  the  inferior  vena  cava  and  portal  vein  contaminates  but  slightly 
the  greater  volume  of  pure  placental  blood.  According  to  common  belief, 
the  blood  from  the  inferior  vena  cava  is  directed  by  the  valve  of  that  vein 
across  the  right  atrium  and  through  the  foramen  ovale  into  the  left 
atrium  (following  the  path  of  the  sounds  in  Figs.  172  to  174),  which,  before 
birth,  receives  little  venous  blood  from  the  lungs.  This  purer  blood  of  the 
left  atrium  then  enters  the  left  ventricle  and  is  driven  out  through  the 
aorta,  to  be  distributed  chiefly  to  the  head  and  upper  extremities. 


202 


THE  VASCULAR  SYSTEM 


The  venous  blood  of  the  superior  vena  cava  is  supposed  to  flow  from 
the  right  atrium  into  the  right  ventricle,  whence  it  passes  out  by  the  pul- 
monary artery.  A small  amount  is  conveyed  to  the  lungs,  but,  as  the 
fetal  lungs  do  not  function,  most  of  it  enters  the  dorsal  aorta  by  way  of  the 
ductus  arteriosus.  Since  the  ductus  is  caudal  to  the  origin  of  the  subcla- 
vian and  carotid  arteries,  its  less  pure  blood  is  distributed  to  the  trunk, 
viscera,  and  lower  extremities.  The  placental  circuit  is  completed  through 
the  hypogastric,  or  umbilical  arteries. 

In  spite  of  the  apparent  anatomical  arrangement  in  the  heart  to 
prev'ent  the  mixing  of  pure  and  impure  blood,  actual  experiments  indicate 


Fig.  199. — Diagram  of  the  circulation  after  birth  (Heisler).  Obliterated  fetal  passages  are 

indicated  by  roman  type. 

that,  contrary  to  the  prevalent  view,  there  is  thorough  mingling  of  the 
blood  which  enters  the  right  atrium  through  the  two  caval  veins.  Hence, 
there  can  be  no  difference  in  the  quality  of  the  blood  distributed  to  the 
various  parts  of  the  body.  Circulatory  efficiency  must  then  depend  on  the 
relatively  large  quantity  of  swiftly  moving  blood. 

Changes  at  Birth. — When  the  lungs  become  functional,  the  placental 
circulation  ceases  quickly.  This  transfer  of  the  seat  of  oxygenation  not 
only  changes  the  character  of  the  blood  in  many  vessels  but  throws 
important  fetal  vessels  and  parts  into  disuse  (Fig.  199).  In  general, 
physiological  occlusion  follows  immediately  but  anatomical  obliteration 
is  slower. 


THE  LYMPHATIC  SYSTEM 


203 


The  large  amount  of  blood  returned  to  the  heart  from  the  functional 
lungs  equalizes  the  pressure  in  the  two  atria  (p.  181).  As  a result,  the 
septum  primum,  or  valve  of  the  forcuucn  ovale,  is  pressed  against  the  septum 
secundum,  thereby  closing  the  foramen.  Eventually,  the  two  septa 
fuse — in  one-third  of  all  cases  within  three  months,  in  three-fourths  by 
maturity  (p.  185). 

The  ductus  arteriosus  also  ceases  to  function,  as  all  the  blood  from  the 
pulmonary  arterial  trunk  is  conveyed  to  the  expanded  lungs.  In  four  out  of 
five  cases  the  ductus  becomes  impervious  within  three  months  and  persists 
as  a solid,  fibrous  cord,  the  Ugamentum  arteriosum. 

The  umbilical  vessels  contract  and  their  lumina  are  obliterated  by 
fibrous  invasion.  The  process  advances  proximad  during  the  first  two  or 
three  months  of  postfetal  life.  The  cord-like  vein  is  persistent  as  the 
Ugamentum  teres  of  the  liver;  the  arteries  become  the  lateral  umbilical 
ligaments. 

The  ductus  venosus  likewise  atrophies,  and,  within  two  months,  trans- 
forms into  the  fibroiis  Ugamentum  venosum,  embedded  in  the  wall  of  the 
liver. 

THE  LYMPHATIC  SYSTEM 

The  lymphatics  originate  independently  of  blood  Y’essels  from  discrete 
mesenchymal  spaces  which  become  lined  with  an  endothelium  of  trans- 
formed border  cells.  Temporary  venous  connections  are  now  generally 
believed  to  be  acquired  secondarily.  By  the  progressive  fusion  and 
budding  of  such  local  anlages,  the  lymphatic  system  grows  to  its  final 
form. 

The  first  plexus  of  lymphatic  capillaries  is  distributed  along  the  primi- 
tive, main  venous  trunks.  The  dilatation  and  coalescence  of  this  network 
at  definite  regions  gives  rise  to  five  lymph  sacs  (Fig.  200):  (i,  2)  Paired 
jugular  sacs  appear  in  10  mm.  embryos,  lateral  to  the  internal  jugular 
veins.  (3)  At  23  mm.,  the  unpaired  retroperitoneal  sac  develops  at  the  root 
of  the  mesentery,  adjacent  to  the  suprarenal  glands,  and  the  cisterna 
chyli  also  differentiates  (4,  5).  Paired  posterior  sacs  arise  in  relation 
to  the  sciatic  veins  of  embryos  24  mm.  long.  All  these  sacs  at  first  contain 
blood  which  they  soon  discharge  into  neighboring  veins,  thereupon  losing 
their  venous  connections.  With  relation  to  the  lymph  sacs  as  centers, 
the  thoracic  duct  (at  30  mm.)  and  the  peripheral  lymphatics  develop.  Thus, 
lymphatic  vessels  grow  to  the  head,  neck,  and  arm  from  the  jugular  sacs; 
to  the  hip,  back,  and  leg  from  the  posterior  sacs,  and  to  the  mesentery  from 
the  retroperitoneal  sac.  The  jugular  sacs  alone  acquire  permanent 
connections  with  the  internal  jugular  veins  that  are  later  utilized  by  the 
thoracic  and  right  lymphatic  ducts.  The  various  sacs  themselves  are 
eventually  replaced  by  chains  of  lymph  glands. 


204 


THE  VASCULAR  SYSTEM 


Lymph  Glands.  - -Paired  lymph  glands  appear  during  the  third 
month,  first  in  the  axillary,  iliac,  and  maxillary  regions  (Fig.  200).  Those 
replacing  the  lymph  sacs  develop  later.  Primitive  sinuses,  with  simple 
connective-tissue  septa,  mark  the  primary  stage  of  development.  Ordi- 
narily it  has  been  believed  that  the  sinuses  represent  lymphatic  plexuses, 
but  recent  investigators  (Downey,  1922)  claim  they  are  channels  in  the 
reticulum,  originating  as  clefts  in  the  mesenchyme  and  acquiring  secondary 


Fir,.  200. — Profile  reconstruction  of  the  primitive  lymphatic  system  in  a human  embryo  of 
two  months  (redrawn  after  Sabin).  X 3. 

lymphatic  connections.  Lymphocytes  collect  in  the  stroma,  forming 
cortical  nodules  which  become  associated  with  blood  capillaries  and  after 
birth  acquire  germinal  centers  (Fig.  201  A).  The  peripheral  sinus  organizes 
and  connects  with  afferent  and  efferent  lymphatics;  the  central  sinuses 
cut  the  lymphoid  tissue  into  medullary  cords  (Fig.  201  B).  The  connective 
tissue  differentiates  into  a fibrous  capsule  from  which  trabeculce  dip  into 
the  gland. 

Hemal  (Hemolymph)  Glands.-  -The  orgin  of  hemal  glands  is  traced 
by  Meyer  (1917)  to  condensations  of  mesenchyme  which  develop  in  rela- 
tion to  blood  vessels,  not  lymphatics.  The  peripheral  sinus  arises  inde- 
pendently; its  vascular  connections  are  secondary. 


A 


Afferent  lymphatic  vessels 


Living  nf  sinus 


Peripheral  lymph  sinus 


Lymphatic  vessel  Blood  vessels 


Lymphatic  vessel 


B 

Afferent  lymphatic  vessels 


Efferent  lymphatic  vessels 

Fig.  201 . — Diagrams  representing  four  stages  in  the  development  of  lymph  glands.  The  earlier 
stages  are  shown  on  the  left  side  of  each  figure  (Lewis  and  Stohr). 


THE  LYMPHATIC  SYSTEM 


205 


The  Spleen. — Embryos  of  9 mm.  exhibit  a swelling  on  the  left  side  of 
the  dorsal  mesogastrium,  near  the  dorsal  pancreas  (Fig.  202  ^d).  The 
thickening  is  due  to  a temporary  proliferation  and  invasion  of  mesothelial 
cells  into  the  underlying  mesenchyme,  which,  meanwhile,  has  also  under- 
gone local  enlargement  and  vascularization.  These  cells  from  the  perito- 
neal epithelium  give  rise  to  a large  part,  at  least,  of  the  future  spleen.  The 
union  of  the  splenic  anlage  with  the  mesogastrium  (Fig.  202  B)  is  ultimately 
reduced  to  a narrow  band. 

At  first  the  blood  vessels  constitute  a closed  system.  The  peculiar 
adult  circulation  is  acquired  relatwely  late.  Lymphoid  tissue  first  appears 
as  ellipsoids  about  the  smallest  arteries  in  fetuses  of  four  months.  At 


Fig.  202. — Developmental  stages  of  the  human  spleen  (redrawn  from  Kollman  and  Tonkoff). 

A,  10.5  mm.;  B,  20  mm. 

seven  months,  the  ovoid  splenic  corpuscles  form  nodules  about  the  larger 
arteries.  The  capsule,  trabeculce,  and  reticulum  differentiate  from  the 
cells  of  the  common  anlage.  During  the  last  half  of  fetal  life,  red  blood 
corpuscles  are  developed  actively  in  the  splenic  capillaries. 

The  Glomus  Coccygeum. — The  coccygeal  body  develops  from  the 
wall  of  the  middle  sacral  artery.  It  appears  at  the  apex  of  the  coccyx 
in  the  third  month,  and,  during  the  fourth  month,  is  an  encapsulated 
cluster  of  polyhedral  cells.  Later,  it  becomes  lobulated  by  the  ingrowth 
of  connective  tissue  trabeculae  and  receives  a rich  vascular  supply.  Affini- 
ties are  obscure,  but  at  no  time  does  it  resemble  chromaffin  tissue,  as  is 
often  stated. 

Tonsils  and  Thymus. — ^For  their  development  see  p.  100 


CHAPTER  X 


THE  SKELETAL  SYSTEM 
I.  Histogenesis  of  the  Supporting  Tissues 

Connective  tissue,  cartilage,  and  bone  all  differentiate  from  that 
type  of  diffuse  mesoderm  known  mesenchyme  (Fig.  3).  Mesenchyme 
arises  directly  from  the  primitive  streak,  and  secondarily  from  meso- 
dermal segments  and  the  lateral  somatic  and  splanchnic  layers  (Fig. 
211).  It  is  a spongy  mesh  work  composed  of  branching  and  anastomosing 
cells;  between  these  occur  open  spaces  filled  with  a ground  substance  of 
coagulable  fluid.  In  early  embryos  the  mesenchyme  constitutes  an 
unspecialized  packing  material  between  the  external  and  internal  epithelia 
(Fig.  212),  but  it  soon  differentiates  into  various  tissues  and  organs  (p.  7). 
(jf  these,  the  inert  supporting  tissues  are  peculiar  in  that  a fibrous,  hyaline, 
or  calcified  matrix  forms,  which  becomes  bulkier  than  the  persisting  cellular 
elements.  In  each  type  the  origin  of  such  matrix,  whether  inter-  or 
intracellular,  is  disputed. 

Connective  Tissue 

Discordant  views  exist  as  to  the  precise  manner  in  which  connective- 
tissue  fibers  differentiate.  Some  maintain  that  the  fibrils  develop  within 
the  cytoplasm  of  mesenchyme  cells  and  are  subsequently  extruded  into 
the  adjoining  matrix.  A modification  of  this  theory,  proposed  by  Mall, 
traces  the  origin  of  the  fibrils  to  a hyaline,  ectoplasmic  layer  of  the  syn- 
cytial mesenchyme;  this  layer  then  transforms  into  matrix,  while  the 
nuclei  and  granular  endoplasm  remain  unaffected  as  definitive  connec- 
tive-tissue cells. 

A rival  theory,  strongly  su]>ported  of  late,  interprets  the  primitive 
matrix  as  a lifeless,  gelatinous  ground  substance,  secreted  by  the  mesen- 
chyme. In  it  fibers  are  formed  by  a gradual  process  of  organization,  which, 
according  to  Baitsell  (iq2i),  is  structurally  identical  with  the  transforma- 
tion of  a plasma  clot  into  fibrin. 

Reticular  Tissue. — Except  for  the  jelly-like  mucous  tissue  of  the 
umbilical  cord,  reticular  tissue  departs  least  from  the  embryonal  type. 
The  fine  reticular  fibrils  remain  embedded  within  the  cytoplasm  of  the 
cells  (Downey,  1922). 

White  Fibrous  Tissue. -^The  differentiation  of  this  tissue  may  be 
divided  into  two  phases:  (i  j a prefibrous  stage,  marked  by  the  appearance 

206 


CONXECTIVE  TISSUE 


207 


of  fibrils  resembling  those  of  reticular  tissue  (Fig.  203,  at  top);  (2)  the 
fibrils  take  the  form  of  parallel  bundles  and  are  converted,  through  a 
chemical  change,  into  typical  white  fibers  (Fig.  203,  at  middle).  The 
early,  spindle-shaped  cells  transform  into  the  several  types  characteristic 


Fig.  203. — The  differentiation  of  white  fibers  in  the  Fig.  204. — The  differentia- 

skin  of  a 5 cm.  pig  embryo  (after  Mall).  X 270.  tion  of  elastic  fibers  in  the  um- 

bilical cord  of  a 7 cm.  pig  embryo 
(after  Mall).  X 270. 


of  the  adult.  In  areolar  tissue,  the  bundles  of  white  fibers  are  inter- 
woven to  form  a meshwork;  in  tendon,  ligaments,  and  fascias  they  are 
arranged  in  compact,  parallel  fascicles. 

D 


Elastic  Tissue. — Yellow  elastic  fibers  develop  in  association  with  the 
white  variety  (Fig.  204).  They  originate  singly  after  the  general  manner 
of  white  fibers,  but  may  coalesce,  as  in  the  fenestrated  membranes  of 
arteries. 

Adipose  Tissue.—  Certain  of  the  mesenchymal  cells  give  rise,  not  to 
fibroblasts,  but  to  fat  cells.  They  secrete  within  their  cytoplasm  droplets 
I of  fat  which  increase  in  size  and  become  confluent  (Fig.  205).  Finally, 

i a single  globule  fills  the  cell,  and  the  nucleus  and  C3’'toplasm  are  pressed 


Fig.  206. — Two  interpretations  of  the  development  of 
cartilage  (Lewis  and  Stohr).  .1,  Studnicka;  B,  Mall. 


Mes.  Precartilaqe  Cariilage. 


THE  SKELETAL  SYSTEM 


2oS 

to  the  periphery.  Fat  cells  are  most  numerous  along  the  course  of  blood  ' 
vessels  in  areolar  tissue  and  a])pear  first  during  the  fourth  month. 

In  several  locations  there  are  groups  of  distinctive,  granular  lipo- 

blasts,  termed  adipose  glands,  but  at  infancy  they  become  indistinguish-  I 

able  from  the  ordinary  fat  cells.  15 

'i 

CARTILAGE  \ 

A preliminary  stage  in  the  development  of  cartilage  begins  as  early  ^ 

as  the  fifth  week  with  the  enlargement  of  mesenchymal  cells  to  form  a ’ 

compact,  cellular  ti.ssue,  designated  prccartilage  (Fig.  206).  The  origin  ^ 
of  mat  fix  is  interpreted  in  two  ways:  (i)  Some  claim  that  it  appears 
between  the  cells  from  thickened  and  transformed  ectoplasmic  walls 
(Fig.  206  A).  (2)  According  to  Mall,  mesenchymal  cells  give  rise  first 

to  an  ectoplasm  in  which  fibrillce  develop.  Next,  the  cells  increase  in 
size  and  are  gradually  extruded  until  they  lie  in  the  intercellular  spaces 
(Fig.  206  B).  Simultaneously,  the  ectoplasm  undergoes  both  a chemical 
and  structural  change  and  is  converted  into  the  hyaline  matrix  peculiar 
to  cartilage. 

The  matrix  of  hyaline  cartilage  remains  homogeneous.  In  fibro-  i 
cartilage,  white  fibers  also  are  dejjosited  within  the  matrix,  in  elastic  f 
cartilage,  yellow  elastic  fibers.  Cartilage  grows  both  internally  and  • 
externally.  Interstitial  growth  results  from  the  proliferation  of  cartilage 
cells  and  the  production  of  new  matrix  by  them.  Ap positional  growth 
takes  place  through  the  mitotic  activity  of  the  connective-tissue  sheath,  y 
the  perichondrium;  its  inner  cells  are  transformed  into  young  cartilage  cells. 

BONE 

Bone  begins  to  appear  after  the  sixth  week.  There  are  two  types: 
the  membrane  bones  of  the  face  and  cranium  which  develop  directly  within 
fibrous  sheets,  and  the  cartilage  bones  which  replace  the  earlier  cartila- 
ginous skeleton.  The  mode  of  histogenesis,  however,  is  identical  in  each. 
Bone  matrix  forms  through  the  activity  of  specialized,  connective-tissue 
cells,  named  osteoblasts  (bone-formers).  First,  fibrilla2  and  then  inter- 
fibrillar  lime  salts  differentiate  (Fig.  207).  Whether  these  constituents  '. 
are  transformed  ectoplasm  or  intercellular  deposits  is  debated. 

Development  of  Membrane  Bone. — The  flat  bones  of  the  face 
and  cranial  vault  are  preceded  by  connective-tissue  membrane.  At  one  ■ I 
or  more  central  points  intramembranous  ossification  begins.  Such  centers 
of  ossification  are  characterized  by  the  appearance  of  osteoblasts  which  ; 

promptly  deposit  bone  matrix  in  the  form  of  spicules  (Fig.  207  A).  These  | 

unite  into  a mesh  work  of  trabeculjc  that  spreads  radialty  in  all  directions.  :i 
Since  the  osteoblasts  are  arranged  in  an  epithelioid  layer  upon  the  surface  J 
of  a spicule,  the  latter  grows  both  in  thickness  and  at  its  tip  (Fig.  207).  ; !| 


BOXE 


2og 


As  the  matrix  is  progressively  laid  down,  some  osteoblasts  become  trapped 
and  remain  imprisoned  as  bone  cells;  these  are  lodged  in  spaces  termed 
lacnncc. 

Somewhat  later,  the  mesenchyme  next;  the  flat  surfaces  of  the  spongy 
plate  thus  formed  condenses  into  a fibrous  membrane,  the  periosteum 
(Fig.  207  B).  Osteoblasts  arise  on  its  inner  surface  and  deposit  parallel 
lamellte  of  compact  bone.  This  process  is  known  as  periosteal  ossification. 


Bone  matrix 


A 

Bone  matrix 


Bone  cell 


C' 


■ mil 


Osteoclast 
in  bone  matrix 


Fig.  207. — Two  stages  in  the  development  of  bone.  .4,  Section  through  the  frontal  bone 
of  a 20  mm.  pig  embryo  (after  Mall).  X 270.  B,  Section  through  the  periosteum  and  bone 
lamellse  of  the  mandible  of  a three-months’  fetus  (Prentiss).  X 325. 


Periosteum 


Osteoblast 


In  such  a manner  are  developed  the  dense  inner  and  outer  tables,  joined 
by  the  spongy  diploe  already  described. 

Much  bone  that  is  first  formed  is  provisional,  and  so  is  resorbed  and 
replaced  in  varying  degrees  as  the  bone  grows  and  assumes  its  final  model- 
ling. At  this  time,  large,  multinucleate  cells  appear  upon  the  surface  of 
the  bone  matrix  (Fig.  207  B).  These  giant  cells  are  named  osteoclasts, 
that  is,  bone  destroyers.  There  is,  however,  no  positive  evidence  that 
the  osteoclasts  are  responsible  for  bone  dissolution;  more  likel}^  they 
are  degenerating,  fused  osteoblasts  and  freed  bone  cells  (Arey,  1920). 
The  open  spaces  of  spongy  bone  are  filled  with  derivatives  of  the  mesen- 

14 


2 10 


THE  SKELETAL  SYSTEJI 


chyme.  Such  reticular  tissue,  fat  cells,  blood  vessels,  and  developing 
blood  cells  constitute  the  red  hone  marroiv. 

Development  of  Cartilage  Bone. — The  shape  of  a cartilage  bone  is 
determined  by  the  transitory  cartilage  model  which  precedes  it  (Fig.  209). 
The  chief  peculiarity  of  this  method  of  bone  formation  is  the  preliminary 
destruction  of  the  cartilage.  For  this  reason,  these  skeletal  elements  are 
often  designated  replacement  bones.  Thereafter,  the  course  of  events 
is  essentially  as  in  the  development  of  a membrane  bone.  Ossification 
occurs  both  within  the  eroded  cartilage  and  peripherally  beneath  its  peri- 
chondrium (Fig.  208).  In  the  first  case,  the  process  is  intracartilaginous 
or  endochondral,  in  the  second  instance,  perichondral,  or  better,  periosteal. 

Endochondral  Bone  Formation. — In  the  center  of  the  cartilage  the 
cells  enlarge,  become  arranged  in  characteristic  radial  rows,  and  lime  is 
deposited  in  their  matrix  (Fig.  208).  The  cartilage  cells  and  part  of  the 
calcified  matrix  then  disintegrate  and  disappear,  thereby  forming  pri- 
mordial marrow  cavities.  This  destruction  apparently  is  caused  by  the 
vascular  primary  marrow  tissue  which  simultaneously  invades  the  cartilage. 
It  arises  from  the  inner,  cellular  layer  of  the  perichondrium  and  burrows 
into  the  cartilage  in  bud-like  cords.  Such  eruptive  tissue  gives  rise  to 
osteoblasts  and  bone  marrow  which  occupy  the  primordial  marrow  cavi- 
ties. The  osteoblasts  first  de])osit  matrix  directly  upon  persisting  spicules 
of  cartilage,  hence  endochondral  bone  is  spongy.  Similarly,  the  hitherto 
intact  regions  of  the  cartilage  undergo  progressive  invasion,  destruction, 
and  replacement  until  eventually  the  entire  cartilage  is  superseded  by 
cancellous  bone. 

Periosteal  Bone  Formation. — While  the  foregoing  changes  are  occurring 
within  the  cartilage,  compact  bone  develops  about  it  (Fig.  208).  This 
process  is  identical  with  the  formation  of  the  tables  of  the  flat  bones,  and 
likewise  is  due  to  the  activity  of  the  inner  osteogenetic  layer  of  the  peri- 
chondrium which  now  converts  directly  into  the  periosteum.  Those 
bone  lamelkv  deposited  about  blood  vessels  that  course  in  hollowed  grooves 
are  concentrically  arranged  into  Flaversian  systems. 

Growth  of  Bones. — Flat  membrane  bones  increase  in  lateral  extent 
by  continued  marginal  ossification  from  osteoblast-rich  connective  tissue 
at  the  site  of  the  later  sutures.  Both  cartilage  and  membrane  bones 
grow  in  thickness  by  the  further  deposition  of  periosteal  matrix.  In 
a long  bone,  this  superficial  accretion  is  accompanied  by  a central  resorp- 
tion which  destroys  not  only  the  endochondral  osseous  tissue  but  also  much 
of  the  earlier  periosteal  layers.  As  a result,  cancellous  bone,  and  its 
associated  red  bone  marrow,  persist  only  at  the  ends,  whereas  in  the  middle 
region  an  extensive  open  cavity  develops.  The  latter  is  filled  with  yellow 
bone  marrow,  composed  chiefly  of  fat  cells. 


Zone  of  cartilage  erosion 
and  endochondral  ossification 


^ Zone  of  calcified  cartilage 
I (Cells  swollen  and  in  rows) 


■j! 


/ Zone  if  unmodified  cartilage 


- Level  of  diarthroidal  joint  anlage 


Periosteal  bone 


Marrow  cavity 


Endochondral  hone  deposited 
OH  remains  of  cartilage 


Position  of  later  epiphysis 


Fig.  208.  Cartilage  bone  development  as  illustrated  in  the  finger  of  a five-months’  fetus 
(Sobotta).  X 15.  Longitudinal  section. 


J 


BONE 


2II 


Most  bones,  especially  those  preformed  in  cartilage,  have  more  than 
one  center  of  ossification  (Fig.  209).  In  all,  there  are  over  8co  such  centers, 
half  of  which  first  appear  after  birth.  On  the  average,  therefore,  there  are 
four  centers  to  each  mature  bone.  Many  cartilage  bones,  such  as  occur 
in  the  extremities,  vertebree  and  ribs,  lack  periosteum  on  their  articular 
surfaces.  The  consequent  inability  to  increase  in  length  by  ordinary  means 
has  led  to  an  interesting  adaptation.  The  cartilage  at  either  end  grows 
rapidly,  and  progressively  ossifies,  but  sometime  between  birth  and 
puberty,  or  even  later,  osteogenetic  tissue  invades  these  cartilages  and 
secondary  ossification  centers,  the  epiphyses,  are  established  (Fig.  209  C, 
D).  Both  surfaces  of  the  intervening  cartilaginous  plate  continue  to 
develop  new  cartilage  as  long  as  the  bone  lengthens,  and  this  in  turn  is 
steadily  replaced  by  bone  matrix.  Finally,  when  the  adult  length  is 
attained,  the  cartilage  ceases  proliferation,  ossifies,  and,  the  epiphyses  are 


Fig.  209. — Diagrams  to  illustrate  the  method  of  growth  in  a long  bone  (Prentiss). 


firmly  united  to  the  central  mass.  The  so-called  epi physeal  lines  mark  this 
union. 

Joints. — The  joints  occur  at  regions  where  the  original  mesenchyme 
fails  to  differentiate  into  skeletal  elements.  Such  articulations  include 
two  general  groups;  (i)  synarthroses,  in  which  little  movement  is  allowed; 
(2)  diarthroses,  or  freely  movable  joints. 

In  joints  of  the  synarthroidal  type,  the  mesenchyme  differentiates 
into  a uniting  layer  of  connective-tissue  {suture,  syndesmosis)  or  cartilage 
{synchondrosis) . 

Diarthroidal  joints  are  characterized  by  a prominent  joint  cavity, 
between  the  movable  skeletal  parts,  and  a ligamentous  capside  at  the 
periphery  (Fig.  210  B).  The  joint  cavity  arises  from  a cleft  in  the  open 
mesenchyme;  the  capsule  from  the  denser  external  tissue  continuous  with 
the  periosteum  (Figs.  208  and  210).  The  cells  on  the  inner  surface  of  the 
capsule  flatten  into  the  epithelioid  synovial  membrane.  Ligaments  or 
tendons  which  apparently  course  through  the  adult  joint  cavities  represent 


212 


THE  SKELETAL  SYSTEM 


secondary  invasions,  covered  with  reflexed  synovial  membrane,  and  hence 
are  really  external  to  the  cavity.  Sesamoid  bones  develop  in  relation  to 
tendons,  and,  usually,  joints;  they  commonly  arise  in  the  substance  of 
the  primitive  joint  capsule  and  may  exhibit  a cartilaginous  stage. 


Cartilage 
Joint  cleft 
Perichondrium 
Mesenchyme 


Marrmv  cav 

Synovial  membrane 

Joint  cavity 

Artictdar  cartilage 
Joint  ca psnle 

Spongy  bone 
Periosteum 
Compact  bone 


Fi('..  2 10. — Stages  in  the  development  of  a diarthroidal  joint. 


II.  MORPHOGENESIS  OF  THE  SKELETON 

The  skeleton  comprises:  (i)  the  axial  skeleton  (skull,  vertebrae,  ribs, 
and  sternum),  and  (2)  the  appendicular  skeleton  (pectoral  and  pelvic  gir- 
dles and  the  limb  bones).  Except  for  the  flat  bones  of  the  face  and  cranial 
vault,  the  l)ones  of  the  mammalian  skeleton  exhibit  first  a blastemal,  or 
membranous  stage,  next  a cartilaginous  phase,  and  Anally  a permanent, 
osseous  condition.  A comparalole  ascending  series  occurs  among  adult 
chordates  of  the  present  day.  It  seems  that  the  bones  of  the  higher  verte- 
brates that  are  descended  from  the  cartilaginous  skeleton  of  Ashes  pass 
through  a reminiscent  cartilaginous  stage,  whereas  those  additional  bones 
made  necessary  by  the  increased  size  of  the  brain  develop  directly  in 
membrane. 

THE  AXIAL  SKELETON 

The  primitive  axial  support  of  all  vertebrates  is  the  notochord,  or 
chorda  dorsalis,  the  origin  of  which  has  been  traced  on  ])p.  40  and  42.  The 
notochord  constitutes  the  only  skeleton  of  Amphioxus,  whereas  in  Ashes 
and  amphibians  it  is  replaced  in  part,  and  in  higher  animals  almost  entirely, 
by  the  permanent  axial  skeleton.  Among  mammals,  this  supporting  rod  is 
transient,  except  at  the  intervertebral  discs  where  it  persists  as  the  nuclei 
pitlposi. 

The  axial  skeleton  differentiates  from  mesenchyme,  most  of  which 
comes  from  the  adjacent  pairs  of  mesodermal  segments.  Toward  the 


THE  AXIAL  SKELETON 


213 


Somatic  7nesoderm 


Splanchnic 

7)t€Soderm 


Fig.  21 1. — Transverse  section  of  a 4.5  mm.  human  embryo,  showing  the  development  of  the 
sclerotomes  (Kollmann).  X about  300. 


Spinal  ganglion 


Dermatome  (?) 


M votome 


Spinal  nerve 


Arm  bud 


Proliferating  cells  of 
myotome 


Mesonephric  duct 

Mesonephric  tubule  and 
glomerulus 

Ccelom 
Somatic  mesoderm 


‘S  ® a ®9  4 

s'a  Q 
6 Q 0a 

Of  99  I 

® Q 


Fig.  212. — Transverse  section  of  a 10.3  mm.  monkey  embryo,  showing  the  sclerotome,  myotome 
and  dermatome  (Kollmann).  A,  aorta;  *,  sclerotome. 


214 


THE  SKELETAL  SYSTEM 


end  of  the  third  week,  their  temporary  cavities  fill  with  diffuse,  spindle- 
shaped  cells,  derived  from  the  surrounding  walls  (Fig.  21 1).  The  median 
side  of  the  segment  then  opens  and  its  mesenchymal  content,  designated  a 
sclerotome,  migrates  mesad  (Fig.  212).  The  sclerotomes  are  the  anlages  of 
verte!:>ra2  and  ribs. 

The  Vertebrae  and  Ribs.- -The  sclerotomic  mesenchyme  comes  to  lie 
in  paired  segmental  masses  on  either  side  of  the  notochord,  separated  from 
similar  masses  before  and  behind  by  the  intersegmenial  arteries  (Fig.  212). 
In  embryos  of  about  4 mm.,  each  sclerotome  differentiates  into  a caudal, 
compact  portion  and  a cranial,  less  dense  half  (Fig.  213  A).  From  the 
caudal  portions,  horizontal  tissue  masses  now  grow  toward  the  median 
plane  and  enclose  the  notochord,  thus  establishing  the  body  of  each  verte- 
bra (Figs.  123  and  214).  Similarly,  dorsal  extensions  pass  dorsad  around 
the  neural  tube  to  form  the  vertebral  arch,  and  ventro-lateral  outgrowths 
constitute  the  costal  processes.  The  looser  tissue  of  the  cranial  halves 


Fig.  213. — Frontal  sections  through  the  left  mesodermal  segments  of  human  embryos. 
A,  at  about  4 mm.,  showing  the  differentiation  of  the  sclerotomes  into  less  dense  and  denser 
regions;  B , at  about  5 mm.,  illustrating  the  union  of  the  halves  of  successive  sclerotomes  to  form 
the  anlages  of  the  vertebne. 

also  grows  mesad  and  fills  in  the  intervals  between  successive  denser 
regions. 

The  denser,  caudal  half  of  each  sclerotomic  mass  presently  unites  with 
the  less  dense,  cranial  half  of  the  sclerotome  next  caudad  to  form  the 
anlages  of  the  definitive  vertebra:  (Fig.  213  B).  Mesenchymal  tissue, 
filling  the  new  intervertebral  fissure  thus  formed,  gives  rise  to  the  inter- 
vertebral discs.  Since  a vertebra  is  formed  from  parts  of  two  adjacent 
sclerotomes,  it  is  evident  that  the  intersegmental  artery  must  now  pass 
over  the  body  of  a vertebra,  and  the  myotomes  and  vertebra  alternate  in 
position. 

Following  this  blastcmal  stage,  centers  of  chondrification  appear,  two 
centers  in  the  vertebral  body,  one  in  each  half  of  the  vertebral  arch,  and 
one  in  each  costal  process  (Fig.  214).  These  centers  enlarge  and  fuse  into 
a solid  cartilaginous  vertebra.  The  original  union  of  the  costal  processes, 
which  will  give  rise  to  ribs,  with  the  vertebral  body  is  temporary,  for  an 


■T  i ■■■■  Notochord 

Hi' 

I ,>  A nlage  of  vertebra 

it 

'■'■i:.  $... Intervertebral  fissure 
C-i  i 

^—Intersegmental  artery 


B 


THE  AXIAL  SKELETON 


215 


articulation  next  the  head  develops  subsequently.  T ransverse  and  articular 
processes  grow  out  from  the  vertebral  arch,  and  the  rib  cartilages,  having 
in  the  meantime  formed  tubercles,  articulate  with  the  transverse  processes 
somewhat  later.  The  various  ligaments  of  the  vertebral  column  arise  from 
mesenchyme  surrounding  the  vertebree. 

Finally,  at  the  end  of  the  eighth  week,  the  stage  of  ossification  sets  in 
(Fig.  222).  A single  center  appears  in  the  body,  one  in  each  half  of  the 
arch,  and  one  near  the  angle  of  each  rib.  The  replacement  of  cartilage 
by  bone  is  not  completed  until  several  years  after  birth.  At  about  the 
seventeenth  year,  secondary  centers  arise  in  the  cartilage  still  covering 
the  cranial  and  caudal  ends  of  the  vertebral  body  and  form  the  disc-like, 
bony  epiphyses.  These  unite  with  the  vertebra  proper  to  constitute  a 
single  mass  at  about  the  twentieth  year. 


Neural  tube 


Fig.  214. — Transverse  section  through  the  blastemal  stages  of  vertebra  and  ribs  from  a 
13  mm.  human  embryo  (partly  after  Bardeen).  X 18.  The  light  areas  are  centers  of 
chondrification. 


While  the  foregoing  account  holds  for  vertebrae  in  general,  a few'  devia- 
tions occur.  When  the  atlas  is  formed,  a body  differentiates  as  w'ell,  but 
it  is  appropriated  by  the  body  of  the  epistropheus  (axis),  thereafter  serving 
as  the  tooth-like  dens  of  the  latter.  The  sacral  and  coccygeal  vertebrae 
represent  types  wdth  reduced  vertebral  arches.  At  about  the  tw'enty- 
fifth  year  the  sacral  vertebrae  unite  to  form  a single  bony  mass,  and  a 
similar  fusion  occurs  between  the  rudimentary  coccygeal  vertebra. 

The  ribs  originate  in  the  costal  processes  wTich  are  ventro-lateral 
outgrow’ths  from  the  vertebral  bodies  (Fig.  214).  Each  has  an  early 
center  of  chondrification  and  ossification  (Fig.  222).  About  puberty, 
two  epiphyseal  centers  appear  in  the  tubercle  and  one  in  the  head.  The 
highest  development  of  ribs  is  realized  in  the  thoracic  region.  In  the 
cervical  region  they  are  short;  their  tubercles  fuse  wdth  the  transverse 
processes  and  their  heads  wfith  the  vertebral  bodies,  thus  leaving  intervals, 
the  transverse  foramina,  through  wTich  the  vertebral  vessels  course.  In 
the  lumbar  region,  the  ribs  are  again  diminutive  and  are  fused  to  the  trans- 


THE  SKELETAL  SYSTEM 


216 

verse  ])rocesses.  The  rudimentary  ribs  of  the  sacral  vertebras  are  repre- 
sented by  flat  plates  which  unite  on  each  side  to  form  a pars  lateralis  of 
the  sacrum,  (.inly  in  the  first  of  the  coccygeal  vertebrae  are  there  traces  of 
ribs. 

The  Sternum.  -Modern  studies  prove  that  the  sternal  anlages  arise 
as  paired  mesenchymal  bands,  with  which  the  first  eight  or  nine  thoracic 
ribs  fuse  secondarily.  After  the  heart  descends  into  the  thorax,  these 
cartilaginous  sternal  bars,  as  they  now  may  be  termed,  unite  in  a cranio- 
caudal  direction  to  form  the  sternum,  at  the  same  time  incorporating  a 
smaller,  mesial  sternal  anlage  (Fig.  215).  Ultimately,  one  or  two  pairs  of 
the  most  caudal  ribs  lose  their  sternal  connections,  the  corresponding 


Fig.  215.— The  sternum  of  a human  fetus  Fig.  216.— Sternum  of  a child,  showing  cent- 
during  the  third  month.  ers  of  ossification. 

portion  of  the  sternum  constituting  the  xiphoid  process  in  part.  At  the 
cranial  end  of  the  sternum  there  are  two  imperfectly  separated  episternal 
cartilages  with  which  the  clavicles  articulate.  These  usually  unite  with  the 
longitudinal  bars  and  contribute  to  the  formation  of  the  manubrium. 
Variations  in  the  ossification  centers  are  not  uncommon,  although  a 
primitive,  bilateral,  segmental  arrangement  is  evident  (Fig.  216).  In 
the  two  cranial  segments,  however,  unpaired  centers  occur. 

The  Skull.  -The  head  skeleton  includes  three  primary  components: 
(i)  the  brain  case;  (2)  capsular  investments  of  the  sense  organs;  (3)  a 
branchial-arch  skeleton,  derived  from  the  peculiar  arches  that  enclose  the 
first  part  of  the  alimentary  tract  in  all  embryos  and  in  adult  fishes  and  tailed 
amphibia  (cf.  p.  77).  Apart  from  exceptions  in  the  third  group,  these 
elements  unite  intimately  into  a composite  mammalian  skull. 

The  notochord  originally  extends  into  the  head  as  far  as  the  pharyn- 
geal membrane.  Not  only  is  the  skull  built  around  it,  but  the  accommoda- 


THE  AXIAL  SKELETON 


217 


tion  of  the  cerebral  hemispheres  has  made  necessary  a prechordal  develop- 
ment which  includes  those  bones  in  front  of  the  sella  turcica. 

The  earliest  anlage  of  the  skull  is  a mass  of  dense  mesenchyme,  which, 
at  the  end  of  the  first  month,  envelops  the  cranial  end  of  the  notochord  and 
extends  cephalad  into  the  nasal  region.  Laterally,  it  forms  wings  which 
enclose  the  neural  tube.  Mesodermal  segments  do  not  form  in  front  of  the 
otocysts,  so,  except  in  the  occipital  region,  where  there  are  indications  of 
the  incorporation  of  three  or  four  vertebrae,  the  skull  is  from  the  first 
devoid  of  segmentation. 

Early  in  the  second  month  chondrification  begins  mesially  in  the  future 
occipital  and  sphenoidal  regions,  and  extends  cephalad  and  to  a slight 


- Interparietal 

Supraoccipital 
--Exoccipital 
Condyle 

—Basioccipital 

Fig.  2 18. — Occipital  bone  of  a human  fetus 
of  four  months.  The  portions  still  carti- 
laginous are  shown  as  a homogeneous  back- 
ground. 


Fig.  217. — The  chondrocranium  of  a 14 
mm.  human  embryo  (Levi  in  McMurrich). 
as,  Alisphenoid;  bo,  basioccipital;  bs,  basisphe- 
noid;  eo,  exoccipital;  ni,  Meckel’s  cartilage;  os, 
orbitosphenoid;  p,  periotic;  ps,  presphenoid:  so, 
sella  turcica;  s,  supraoccipital. 


extent  dorsad.  At  the  same  time,  the  internal  ears  become  invested  with 
cartilaginous  periotic  capsules  which  eventually  unite  with  the  occipital 
and  sphenoidal  cartilages  (Fig.  217).  The  chondrocranium,  as  it  is  termed, 
is  thus  confined  chiefly  to  the  base  of  the  skull,  whereas  the  bones  of  the 
sides,  roof,  and  the  face  are  of  membranous  origin.  Chondrification  also 
occurs  more  or  less  extensively  in  the  branchial  arches. 

In  the  period  of  ossification,  which  now  ensues,  it  beeomes  evident  that 
some  bones  which  are  separate  in  adult  lower  animals  fuse  to  form  com- 
pound bones  in  the  human  skull.  The  sphenoid  and  temporal  bones,  for 
example,  represent  five  primitive  pairs  each.  As  such  components  may 
arise  either  in  membrane  or  cartilage,  the  mixed  nature  of  various  adult 
bones  is  explained. 

A striking  feature  of  the  fetal  skull  is  the  great  relative  size  of  the 
neural  portion.  The  ratio  of  cranial  to  facial  volume  decreases  from  8;  i 
at  birth  to  2.5:1  in  the  adult. 


2i8 


THE  SKELETAL  SYSTEM 


Ossification  of  the  Chondrocranimu . — The  Occipital  Bone. — Ossifica- 
tion begins  in  the  occipital  region  during  the  third  month  (Fig.  222). 
Four  centers  a])pear  at  right  angles  about  the  foramen  magnum  (Fig.  218). 
From  the  ventral  center  arises  the  basilar  (hasioccipital)  part  of  the  future 
bone;  from  the  lateral  centers  the  lateral  {cxocci  pital)  portions  which  bear  the 
condyles;  and  from  the  dorsal,  originally  paired  center,  the  squamous 
{supraocci pital)  part  below  the  superior  nuchal  line.  The  squamous 


Ala  magna  Ala  parva 

i.-Uisphcnoid)  Presphenoid  [Orbilosphcnoid) 


Basisphenoid  process 


Fig.  219. — Sphenoid  bone  of  a human  fetus 
of  nearly  four  months.  Parts  still  cartilaginous 
are  represented  in  stipple. 


Fig.  220. — Ethmoid  bone  of  a human 
fetus  of  four  months. 


Squamosum 


{interparietal)  area  above  that  line  is  an  addition  of  intramembranous 
origin.  These  several  components  do  not  fuse  completely  until  about  the 
seventh  year. 

The  Sphenoid  Bone. — Ten  principal  centers  arise  in  the  cartilage  that 
corresponds  to  this  bone  (Fig.  219):  (i  and  2)  in  each  ala  magna  {ali- 

sphenoid)’,  (3  and  4)  in  each  ala  parva  {orbito- 
sphenoid) ; (5  and  6)  in  the  corpus  between  the 
alee  magnac  (basisphenoid)  (7  and  8)  in  each 
lingula’,  (9  and  10)  in  the  corpus  between  the 
alee  parvae  (presphenoid).  Intramembranous 
bone  also  enters  into  its  composition,  forming 
the  orbital  and  temporal  portion  of  each  ala 
magna  and  the  mesial  laminae  of  each  pterygoid 
Fig.  221.— The  left  temporal  pi'occss  (except  the  hamulus).  Fusion  of  the 
bone  at  birth.  The  portion  of  various  regions  is  Completed  during  the  first 
intracartilaginous  origin  is  repre-  y0g^j- 

suited  in  stipple.  The  Ethmoid  Bonc. — The  ethmoid  cartilage 

consists  of  a mesial  mass,  which  extends  from  the  sphenoid  to  the 
tip  of  the  nasal  process,  and  of  paired  masses  lateral  to  the  olfactory 
fossai.  The  lower  part  of  the  mesial  mass  persists  as  the  cartilaginous 
nasal  septum,  but  ossification  of  the  upper  portion  produces  the 
lamina  per pendicularis  and  the  crista  galli  (Fig.  220).  The  lateral 
masses  ossify  at  first  into  the  spongy  bone  of  the  ethmoidal  labyrinths. 
From  this,  the  definitive  honeycomb  structure  (ethmoidal  cells)  and 


THE  AXIAL  SKELETON 


219 


the  concha;  are  formed  through  evaginations  of  the  nasal  mucous 
membrane  and  the  coincident  resorption  of  bone.  (Similar  invasions 
of  the  mucous  membrane  and  dissolution  of  bone  produce  the 
frontal,  sphenoidal,  and  maxillary  sinuses:  p.  297.)  Fibers  of  the 

olfactory  nerve  at  first  course  between  the  unjoined  mesial  and  lateral 
masses.  Later,  cartilaginous,  and  finally,  bony  trabeculse  surround  these 
bundles  of  nerve  fibers;  as  the  cribriform  plates,  they . interconnect  the 
three  masses. 

The  Temporal  Bone. — Several  centers  of  ossification  in  the  periotic 
capsule  unite  to  form  a single  center  from  which  the  whole  cartilage  is 


0 

c 

c 

1 
P 
i 
I 
a 
I 


Fig. 


Parietal 


^Interparietal 

Basi-  and  exoccipital 
Tympanic 


Vertebra 


Hill  m 


Frontal 


Temporal 
Nasal 
Maxilla 
Mandible 
II  u merits 


-Metacarpal 

-Phalanges 


Radius 

Ulna 


- Fibula 
Tibia 


\ 


- Metatarsal 
■ Phalanx 


222. — The  extent  of  ossification  in  a fetus  of  ii  weeks  (after  Broman). 


X 1.5. 


transformed  into  the  petrous  and  mastoid  portions  of  the  temporal  bone 
(Figs.  221  and  222).  The  mastoid  process  is  formed  after  birth  by  a 
bulging  of  the  petrous  bone;  its  internal  cavities,  the  mastoid  cells. 
are  formed  and  lined  by  the  evaginated  epithelial  lining  of  the  middle 
ear.  The  squamosal  and  tympanic  portions  of  the  temporal  bone  are  of 
intramembranous  origin,  while  the  styloid  process  originates  from  the 
proximal  end  of  the  second,  or  hyoid  branchial  arch. 

Membrane  Bones  of  the  Skull. — From  the  preceding  account  it  is 
evident  that,  although  the  bones  forming  the  base  of  the  skull  arise  chiefly 


220 


THE  SKELETAL  SYSTEM 


in  cartilage,  they  receive  substantial  contributions  from  membrane  bones. 
The  remainder  of  the  sides  and  roof  of  the  skull  is  wholly  of  intramem- 
branous  origin,  each  of  the  parietals  forming  from  a single  center,  the 
frontal  from  paired  centers  (Fig.  222).  At  the  incomplete  angles  between 
the  ])arietals  and  their  adjacent  bones,  union  is  delayed  for  some  time  after 
birth.  These  membrane-covered  spaces  constitute  the  fontanellcs,  or 
‘ soft  spots  ’. 

The  vonier  forms  from  two  centers  in  the  connective  tissue  flanking 
the  lower  border  of  the  lamina  perpendicularis  of  the  ethmoid.  The 
cartilage  of  the  ethmoid  thus  invested  undergoes  resorption.  Single 
centers  of  ossification  in  the  mesenchyme  of  the  facial  region  give  rise  to 


Fig. 


Cricoid  cartilage  ( Meckel's  cartilage  (1) 

Hyoid  cartilage  [lesser  horn)  (II) 

Hyoid  cartilage  (greater  horn)  (III) 
Thyroid  cartilage  (IV  -|-  V) 

-Lateral  dissection  of  the  head  of  a human  fetus,  showing  the  derivatives  of  the 
branchial  arches  (after  Kollmann). 


Malleus  (I) 

Incus  (I) 


Temporal  squama 


Stapes  (II) 


Styloid  process  (II 

Tympanic  ring 
Stylo-hyoid  lig.  (II) 


Mandible 


the  nasal,  lacrimal,  and  zygomatic,  all  pure  membrane  bones.  The 
maxillary  and  palate  bones  are  described  in  the  next  paragraph. 

Branchial-Arch  Derivatives. — The  first  branchial  arch  on  each  side 
forks  into  an  upper  maxillary  and  a lower  mandibular  process  (Fig.  64). 
Cartilage  fails  to  appear  in  the  maxillary  processes,  due  to  accelerated 
development,  hence  the  palate  bones  and  the  maxillcr  arise  directly  in 
membrane  (Fig.  222).  Each  palate  bone  develops  from  a single  center  of 
ossification.  According  to  recent  investigations,  two  centers  contribute 
to  the  formation  of  each  maxilla ; one  gives  rise  to  the  portion  bearing  the 
incisor  teeth,  the  other  to  the  remainder  of  the  maxilla. 

The  entire  core  of  the  mandibular  process  becomes  a cartilaginous  bar, 
Meckel’s  cartilage,  which  extends  proximally  into  the  tympanic  cavity  of 
the  ear  (Figs.  222  and  223).  Alembrane  bone,  developing  distally  in 
the  body  of  the  future  lower  jaw,  encloses  Meckel’s  cartilage  and  the  inferior 


THE  APPENDICULAR  SKELETON 


221 


alveolar  nerve,  whereas  proximally  in  the  ramus  the  membrane  bone 
merely  lies  lateral  to  these  structures — hence  the  position  of  the  adult 
mandibular  foramen.  The  portion  of  Meckel’s  cartilage  invested  by 
bone  disappears,  while  the  cartilage  proximal  to  the  mandibular  foramen 
becomes  in  order,  the  s pheno-mandibular  ligament,  the  malleus,  and  the 
incus  (p.  310  and  Fig.  310). 

Each  second  branchial  arch  enters  into  relation  proximally  with  the 
periotic  capsule.  This  upper  segment  of  the  cartilage  becomes  the  stapes 
and  the  styloid  process  of  the  temporal  bone  (Figs.  223  and  310).  The 
succeeding  distal  portion  is  transformed  into  the  stylo-hyoid  ligament;  it 
connects  the  stjdoid  process  with  the  distal  end  of  the  arch,  which  also 
undergoes  intracartilaginous  ossification  to  form  the  lesser  horn  of  the 
hyoid  bone. 

The  cartilage  of  the  third  branchial  arches  ossifies  and  gives  origin  to 
the  greater  horns  of  the  hyoid  bone,  while  a plate  connecting  the  two 
arches  becomes  its  body. 

The  fourth  branchial  arches  differentiate  into  the  cuneiform  cartilages 
and  most  of  the  thyroid  cartilage. 

The  fifth  branchial  arches  appear  to  contribute  to  the  thyroid  carti- 
lage and  to  form  the  corniculate,  arytenoid,  and  cricoid  cartilages. 

THE  APPENDICULAR  SKELETON 

The  appendicular  skeleton  apparently  is  derived  from  the  unseg- 
mented somatic  mesenchyme,  and  not  from  the  sclerotomes.  In  embryos 
of  9 mm.,  mesenchymal  condensations  have  formed  definite  blastemal 
cores  in  the  primitive  limb  buds  (Figs.  212  and  227).  Following  this 
condition,  the  various  bones  pass  through  cartilaginous  and  osseous  stages. 

The  Upper  Extremity.—  The  clavicle  is  the  first  bone  of  the  skeleton 
to  ossify,  centers  appearing  at  each  end  (Fig.  222).  Prior  to  ossification, 
it  is  composed  of  a peculiar  tissue  which  makes  it  difficult  to  decide 
whether  the  bone  is  intramembranous  or  intracartilaginous  in  origin. 

The  scapula  arises  as  a single  plate  with  two  chief  centers  of  ossi- 
fication (Fig.  222).  An  early  center  forms  the  body  and  spine.  The 
other,  after  birth,  gives  rise  to  the  rudimentary  coraeoid  process,  which  in 
lower  vertebrates  extends  from  the  scapula  to  the  sternum.  Union 
between  the  coracoid  process  and  the  body  is  delayed  until  about  the 
fifteenth  year. 

The  humerus,  radius,  and  ulna  ossify  from  single  primary  centers 
and  two  or  more  epiphyseal  centers  (Figs.  209  and  222). 

In  the  cartilaginous  carpus  there  is  a proximal  row  of  three,  and  a 
distal  row  of  four  elements.  Other  inconstant  cartilages  may  appear, 
and  subsequently  disappear  or  become  incorporated  into  the  carpal  bones. 


222 


THE  SKELETAL  SYSTEM 


The  mctacar pals  and  phalanges  develop  from  single  primary  and  epiphy- 
seal centers. 

The  Lower  Extremity.- — The  cartilaginous  plate  of  the  coxal,  or  hip 
bone  is  at  first  so  placed  that  its  long  axis  is  perpendicular  to  the  vertebral 
column  (Fig.  227).  Later,  it  rotates  to  a position  parallel  with  the  verte- 
bral column,  and  shifts  slightly  caudad  to  come  into  relation  with  the 
first  three  sacral  vertebrae  (Fig.  222).  A retention  of  the  membranous 
condition  in  the  lower  half  of  each  primitive  cartilaginous  plate  accounts 
for  the  obturator  membrane  which  closes  the  foramen  of  the  same  name. 
Three  centers  of  ossification  ajjpear,  forming  the  ilium,  ischium,  and 
pubis.  The  three  bones  do  not  fuse  completely  until  about  puberty. 

The  general  development  of  the  femur,  tibia,  fibula,  tarsus,  metatarsus, 
and  phalanges  is  c|uite  similar  to  that  of  the  corresponding  bones  of  the 
upper  extremity.  The  patella,  like  the  pisiform  of  the  carpus,  is  regarded 
as  a sesamoid  bone;  both  develop  within  tendons. 

Anomalies. — Variations  in  the  size,  shape,  and  number  of  skeletal  parts  are  common. 
Developmental  arrest  and  over- development  are  the  prime  causative  factors.  Variations 
in  the  number  of  vertebra?  (except  cervical)  are  not  infrequent.  The  last  cervical  and  first 
lumbar  vertebra?  occasionally  bear  ribs,  due  to  the  continued  development  of  the  primitive 
costal  processes.  Cleft  sternum  or  cleft  xiphoid  process  represents  an  incomplete  fusion 
of  the  sternal  bars.  Additional  fingers  or  toes  (polydactyly)  may  occur;  the  cause  is  ob- 
scure. More  rarely,  there  is  fusion  between  two  or  more  digits  {syndactyly).  Hare  lip  and 
cleft  palate  are  described  in  an  earlier  chapter  (pp.  yq;  8q). 


CHAPTER  XI 


THE  MUSCULAR  SYSTEM 
I.  THE  HISTOGENESIS  OF  MUSCLE 

The  muscular  system  is  composed  of  specialized  cells,  called  muscle 
fibers;  these  form  a tissue  in  which  contractility  has  become  the  pre- 
dominant function.  The  fibers  are  of  three  types:  (i)  smooth,  found 
principally  in  the  walls  of  the  viscera  and  blood  vessels;  (2)  cardiac, 
forming  the  myocardium  of  the  heart;  (3)  skeletal,  chiefly  attached  to  the 
elements  of  the  skeleton.  Of  these,  cardiac  and  skeletal  muscle  are 
banded  with  cross  stripes ; only  skeletal  fibers  are  under  voluntary  control. 
All  three  differentiate  from  myoblasts  of  the  mesoderm;  the  only  excep- 
tions are  the  smooth  muscles  of  the  iris  and  sweat  glands,  which  are 
ectodermal. 

Smooth  Muscle. — Certain  stellate  cells  of  the  mesenchyme  enlarge 
and  elongate.  The  resulting,  spindle-shaped  cells  remain  attached  to 


Fig.  224. — Stages  in  the  histogenesis  of  smooth  muscle  (adapted  from  (McGill).  .4, 
13  mm.  pig  embryo  ( X 550) : coalescing  granules  give  rise  to  coarse  myoglia  fibrils.  B.  27  mm. 
pig  embryo  (X  850):  both  myoglia  fibrils  and  fine  myofibrils  are  present. 

each  other  by  cytoplasmic  bridges.  In  the  superficial  layer  of  their 
cytoplasm  coalesced  granules  form  coarse,  non-contractile  myoglia  fibrils, 
similar  to  the  primitive  fibrillge  of  connective  tissue  (Fig.  224  A).  Fine 
myofibrils  then  differentiate  uniformly  throughout  the  cytoplasm  of  the 
myoblasts  (Fig.  224  B).  They  increase  in  number  as  development 
proceeds,  while  the  coarse  type  diminishes.  The  cytoplasmic  bridges 
later  give  origin  to  white  connective-tissue  fibers  which  envelop  the 
muscle  cells  and  bind  them  together.  In  older  fetuses  new  muscle  ele- 
ments also  arise  by  mitotic  division  of  existing  fibers  and  by  the  trans- 
formation of  apparent  interstitial  cells. 

Cardiac  Muscle. — The  cardiac  type  of  involuntary  muscle  develops 
from  the  splanchnic  mesoderm  that  invests  the  primitive  heart  tubes 

223 


224 


THE  MUSCULAR  SYSTEjM 


(Fig.  155).  The  cells  of  this  myocardial  anlage  at  first  form  a syncytium 
in  which  myojibrils  differentiate  from  the  linear  union  of  cytoplasmic 
granules  (Fig.  225,  A,  B).  The  myofibrillae  arise  at  the  periphery  of  the 
syncytial  strands  of  cytoplasm  and  soon  extend  long  distances  through 
the  syncytium  (D).  They  multiply  ra])idly  (C)  and  form  alternate  light 
and  dark  bands,  as  in  skeletal  muscle.  The  syncytial  character  of  cardiac 
muscle  })ersists  in  the  adult  and  the  nuclei  remain  central  in  position. 
The  intercalated  discs,  typical  of  adult  cardiac  muscle,  probably  appear 
in  the  early  months  of  fetal  life. 

Skeletal  Muscle.  —All  striated  voluntary  muscle  is  derived  from  the 
mesoderm — ^either  from  portion  of  the  mesodermal  segments  (muscles 
of  the  trunk,  and,  possibly,  limbs),  or  from  the  mesenchyme  (muscles  of 
the  head).  According  to  Bardeen,  the  remainder  of  the  primitive  segment 
not  involved  in  forming  skeletal  tissue  constitutes  the  myotome,  and  its 
cells  become  myoblasts  (Fig.  211).  On  the  contrary,  Williams  finds  that 
in  the  chick  only  the  cells  of  the  dorsal  and  mesial  walls  of  a mesodermal 
segment  comprise  the  myotome  (Fig.  212). 

As  to  the  composition  of  the  individual  muscle  fibers,  there  is  also  a 
difference  of  opinion.  It  is  generally  believed  that  the  myoblasts  elongate, 
and,  by  the  repeated  mitotic  division  of  their  nuclei,  become  multi - 
nucleate.  Godlewski,  however,  holds  that  several  myoblasts  unite  to  form 
a single  muscle  fiber.  At  the  beginning  of  differentiation  the  nuclei  lie 
centrally,  surrounded  by  granular  sarcoplasm  (Fig.  226  A).  These 
granules  become  consolidated  in  rows  as  the  myofibrillce,  which  increase 
in  number  by  longitudinal  splitting  (Fig.  226  B,  C).  The  myofibrillae 
soon  acquire  the  characteristic  transverse  bands,  and  the  individual 
fibrils  become  so  grouped  that,  in  the  third  month,  their  dark  and  light 
stripes  coincide  (Fig.  226  C).  During  development,  the  muscle  fibers 
increase  enormously  in  size,  the  nuclei  migrate  to  the  surface,  and  the 
myofil)rilla}  are  arranged  in  bundles,  or  muscle  columns.  The  fibrils  of 
each  column  are  said  to  result  from  the  longitudinal  splitting  of  single, 
primitive  myofibrils.  For  a time  new  muscle  fibers  arise  also  by  the 
division  of  those  already  formed. 

II.  MORPHOGENESIS  OF  THE  MUSCLES 

The  muscles  of  the  body  are  distributed  in  two  systems ; the  visceral 
musculature,  and  the  skeletal  musculature. 

The  Visceral  Musculature. — This  group  is  associated  chiefly  with 
the  hollow  viscera  and  is  under  the  involuntary  control  of  the  sympathetic 
nervous  sytem.  Except  for  the  striated  cardiac  muscle  in  the  wall  of  the 
heart,  the  visceral  muscles  are  smooth.  Their  commonest  arrangement  is 
in  orderly  sheets  or  interlacing  bundles. 


MORPHOGENESIS  OF  THE  MUSCLES 


The  Skeletal  Musculature. — As  the  name  indicates,  these  striated 
muscles  come  in  intimate  relation  to  the  skeleton.  With  the  exception  of 
those  muscles  attached  to  the  branchial  arches,  they  originate  from  that 
portion  of  mesodermal  segments  designated  a myotome,  or  muscle  plate 
(p.  7;  Figs.  21 1 and  212).  Mesodermal  segments  first  appear  in  the  occi- 
pital region  of  embryos  about  1.5  mm.  long  (Fig.  58),  and  the  full  number 
of  nearly  forty  is  acquired  at  6 mm.(  Fig.  63).  At  the  latter  stage  of  about 
five  weeks,  the  myotomes  first  formed  begin  the  differentiation  of  muscles. 
It  will  be  convenient  to  consider  their  morphogenesis  under  three  divi- 
sions: the  muscles  of  the  trunk,  limbs,  and  head. 


Fig.  225. — The  histogenesis  of  cardiac  muscle  in  a 9 mm.  rabbit  embryo  (adapted  after 
Godlewski).  A,  Linear  arrangement  of  granules;  B,  coalescence  of  granules  into  a fibril;  C, 
fibril  splitting;  £>,  long  fibrils  extending  through  syncytium. 


Fig.  226. — Stages  in  the  histogenesis  of  skeletal  muscle  (after  Godlewski).  .4,  Myoblast 
of  a 13  mm.  sheep  embryo;  B,  myofibrils  in  a myoblast  of  a lo  mm.  guinea  pig  embryo;  C, 
myoblast  with  longitudinally-splitting,  striated  myofibrils  from  an  8.5  mm.  rabbit  embryo. 

Fundamental  Processes. — Although  the  primitive  segmental  arrange- 
ment of  the  myotomes  is,  for  the  most  part,  soon  lost,  their  original 
innervation  by  the  segmental  spinal  nerves  is  retained  throughout  life. 
For  this  reason,  the  history  of  adult  muscles  formed  by  fusion,  splitting, 
or  other  modifications  may  be  traced  with  considerable  certainty. 


226 


THE  MUSCULAR  SYSTEM 


The  changes  occurring  in  the  myotonies  during  the  formation  of  adult 
muscles  are  referable  to  the  operation  of  the  following  factors; 

1.  A change  in  direction  of  muscle  fibers  from  the  original  cranio- 
caudal  orientation  in  the  myotome.  The  fibers  of  but  few  muscles  retain 
their  initial  orientation  parallel  to  the  long  axis  of  the  body. 

2.  A 'migration  of  myotonies,  wholly  or  in  part,  to  more  or  less  remote 
regions.  Thus,  the  latissinius  dorsi  originates  from  cervical  myotonies, 
but  finally  attaches  to  the  lower  thoracic  and  lumbar  vertebrae  and  to  the 
crest  of  the  ilium.  Other  examples  are  the  serratus  anterior  and  the 
trajiezius. 

3.  A fusion  of  portions  of  successive  myotonies.  The  rectus  abdomi- 
nis and  sacro-spinalis  illustrate  this  process. 

4.  A longitudinal  splitting  of  myotonies  into  several  portions. 
Examples  are  found  in  the  sterno-  and  omo-hyoid  and  in  the  trapezius  and 
sterno-niastoid. 

5.  A tangential  splitting  into  two  or  more  layers.  The  oblique  and 
the  transverse  muscles  of  the  abdomen  are  formed  in  this  common  way. 

6.  A degeneration  of  myotomes,  wholly  or  in  part.  By  this  process 
fascias,  ligaments,  and  a poneuroscs  may  be  produced. 

M uscles  of  the  I'runk.- -Ventral  extensions  grow  out  from  the  cervical 
and  thoracic  myotonies  (Fig.  212),  and  a fusion  that  is  well  advanced 
superficially  occurs  between  all  the  myotonies  in  embryos  of  10  mm.  A 
dorsal,  longitudinal  column  of  fused  myotonies,  however,  can  still  be 
distinguished  from  the  sheet  formed  from  the  combined  ventral  prolonga- 
tions (Fig.  227). 

From  the  su])erficial  portions  of  the  dorsal  column  there  arise  by 
longitudinal  and  tangential  splitting  the  various  long  muscles  of  the  back  and 
neck,  innervated  by  the  dorsal  rami  of  the  spinal  nerves  (Fig.  2 28).  The 
deep  portions  of  the  myotomes  do  not  fuse,  but  give  rise  to  the  several 
intervertebral  muscles,  which  thus  retain  their  primitive  segmental 
arrangement. 

ddie  muscles  of  the  neck,  other  than  those  innervated  by  the  dorsal 
rami  and  those  arising  from  the  branchial  arches,  differentiate  from  ventral 
extensions  of  the  cervical  myotomes.  The  muscles  of  the  diaphragm,  which 
in  early  stages  lies  at  this  level,  appear  to  have  a like  origin.  In  the  same 
manner,  the  thoraco-abdominal  muscles  arise  from  the  more  pronounced 
ventral  prolongations  of  the  thoracic  myotomes  that  grow  into  the  body 
wall  along  with  the  ribs  (Fig.  228). 

The  ventral  extensions  of  the  lumbar  myotomes  (except  the  first) 
and  of  the  first  two  sacral  myotomes  do  not  participate  in  the  formation 
of  the  body  wall.  If  they  persist  at  all,  it  is  possible  that  they  contribute 
to  the  formation  of  the  lower  limb.  The  ventral  portions  of  the  third 


MORPHOGENESIS  OF  THE  MUSCLES 


227 


and  fourth  sacral  myotomes  give  rise  to  the  muscles  of  the  perineal 
region. 

Muscles  of  the  Limbs. — It  is  commonly  stated  that  the  muscles 
of  the  extremities  develop  from  buds  of  the  myotomes  which  grow  into 


Fig.  227. — Reconstruction  of  a 9 mm.  human  embryo,  to  show  the  partially  fused  myo- 
tomes and  the  premuscle  masses  of  the  limbs  (Bardeen  and  Lewis).  X 13.  Distally,  in  the 
upper  extremity,  the  radius,  ulna  and  hand  plate  are  disclosed;  in  the  lower  extremity  the  hip 
bone  and  the  border  vein  show. 

the  limb  anlages.  In  sharks  this  is  clearly  the  case,  but  in  birds  and 
mammals  distinct  buds  of  this  sort  do  not  occur.  The  segmental  nerve 
supply  of  the  limb  muscles  of  higher  animals  is  merely  suggestive,  not 
proof,  of  a myotomic  origin.  Nevertheless,  a diffuse  migration  of  cells 
from  the  ventral  portion  of  human  myotomes  has  been  recorded  by 
various  observers,  recently  by  Ingalls.  These  cells  soon  lose  their  epithe- 
lial character  and  blend  with  the  undifferentiated  mesench}mie  of  the  limb 


228 


THE  MUSCULAR  SYSTEM 


buds  (Figs.  212  and  227).  From  this  diffuse  tissue,  which  at  about  9 mm. 
forms  condensed,  premuscle  masses,  the  limb  muscles  are  differentiated, 
the  proximal  ones  being  the  first  to  appear.  The  progressive  differentia- 
tion into  distinct  muscles  reaches  the  level  of  the  hand  and  foot  in  embryos 
of  seven  weeks  (Fig.  228).  The  upper  limbs  naturally  maintain  an 
advance  over  the  lower  throughout  development. 

I\Iiisclcs  of  the  Head. — Distinct  mesodermal  segments  do  not  occur 
in  the  head  region.  It  is  possible,  however,  that  a premuscle  mass,  from 


Fig.  228. — Reconstruction  of  the  superficial  muscles  of  a 20  mm.  human  embryo  (Bardeen  and 

Lewis  in  Bailey  and  Miller).  X 4.5. 

which  the  eye  muscles  of  man  are  developed,  is  comparable  to  three  myo- 
tomic  ^segments  having  a similar  fate  in  the  shark.  The  muscles  of  the 
eyes  are  activated  by  the  third,  fourth,  and  sixth  pairs  of  cerebral  nerves. 

The  remaining  muscles  of  the  head  differ  from  all  other  skeletal  mus- 
cles in  that  they  arise  from  the  splanchnic  mesoderm  of  the  branchial 
arches  and  are  innervated  by  nerves  (visceral)  of  a different  category  than 
those  (somatic)  which  supply  myotomic  muscles  (p.  275).  The  muscles  de- 
rived from  the  several  arches  retain  their  primitive  branchial-arch  innerva- 
tion (Fig.  367).  Hence  it  follows  that  the  mesoderm  of  the  first  branchial 
arch  gives  rise  to  the  muscles  of  mastication  and  to  all  other  muscles 
associated  with  the  (fifth)  trigeminal  nerve.  Similarly,  the  muscles  of 


MORPHOGENESIS  OF  THE  MUSCLES 


229 


expression,  and  other  muscles  supplied  by  the  (seventh)  facial  nerve, 
originate  from  the  second,  or  hyoid  arch.  The  third  arch  appears  to  be  the 
source  of  muscles,  like  the  pharyngeal  constrictors,  which  receive  branches 
of  the  (ninth)  glossopharyngeal  nerve.  The  fourth  and  fifth  arches  shave 
the  (tenth)  vagus  nerve;  it  innervates  their  derivatives,  such  as  the  laryn- 
geal muscles  and  part  of  the  pharyngeal  and  palate  group. 

The  muscles  of  the  tongue  are  supplied  by  the  hypoglossal  nerve,  origi- 
nally a member  of  the  spinal  series.  For  this  reason,  it  is  assumed  that,  at 
least  historically,  they  are  derived  from  myotonies  of  the  occipital  region. 
Yet,  according  to  Lewis,  “there  is  no  direct  evidence  whatever  for  this 
statement,  and  we  are  inclined  to  believe  from  our  studies  that  the  tongue 
musculature  is  derived  from  the  mesoderm  of  the  floor  of  the  mouth.” 

Segmentation  of  the  Vertebrate  Head. — The  vertebrate  head  undoubtedly  consists  of 
fused  segments.  This  was  suggested  to  the  earlier  workers  by  the  arrangement  of  the 
branchial  arches  {hranckiomerism),  by  the  presence  of  supposedly  significant  ‘ neiiromeres' 
in  the  brain  wall  (p.  257),  and  by  the  discovery,  in  the  embryos  of  lower  vertebrates,  of 
so-called  head  cavities,  homologous  with  mesodermal  segments. 

Only  the  first  three  head  cavities  persist;  they  form  the  eye  muscles.  All  the  remain- 
ing muscles  of  the  head  are  derived  from  branchiomeres.  Even  assuming  that  the 
branchiomeres  represent  portions  of  the  primary  head  somites — and  there  are  recent 
observations  which  tend  to  disprove  this — their  segmentation  still  is  not  comparable  to 
that  of  the  trunk,  for  the  branchial  arches  are  formed  by  the  serial  division  of  splanchnic 
mesoderm,  tissue  which  in  the  trunk  never  segments.  The  branchial  arches,  therefore, 
represent  a different  sort  of  metamerism.  From  what  has  been  said,  it  is  evident  that  one 
cannot  compare  the  relation  of  the  cranial  nerves  to  the  branchiomeric  muscles  with  the 
relation  of  a spinal  nerve  to  its  myotomic  muscles.  For  this  reason,  the  cerebral  nerves 
furnish  unreliable  evidence  as  to  the  primitive  number  of  cephalic  segments.  A'arious 
investigators  have  set  this  number  between  eight  and  nineteen. 

Anomalies. — Variations  in  the  form,  position,  and  attachments  of  the  muscles  are 
common.  Most  of  these  anomalies  are  referable  to  the  variable  action  of  the  several 
developmental  factors  listed  on  p.  226. 


ECTODERMAL  DERIVATIVES 


CHAPTER  XII 

THE  INTEGUMENTARY  SYSTEM 

The  contribution  of  ectoderm  to  the  front  part  of  the  oral  and  nasal 
cavities,  and  specifically  to  the  development  of  teeth,  tongue,  and  salivary 
glands,  is  described  in  earlier  chapters.  Here  will  be  presented  the  histo- 
genesis of  the  skin  and  the  development  of  its  accessory  epidermal  struc- 
tures. The  integument  is  a double-layered  organ;  only  its  epithelium  is 
derived  from  eetoderm,  whereas  the  fibrous  corium  is  mesodermal. 

THE  SKIN 

The  Epidermis. — The  embryonic  ectoderm  is  originally  a single  sheet 
of  cuboidal  cells  (Fig.  212),  but,  at  the  end  of  the  first  month,  it  consists  of 


Fig.  229. — Sections  of  the  integument  from  a three-months’ fetus  (Prentiss).  X 440-  -E 
From  the  neck,  showing  at  the  right  a two-layered  epidermis  and  at  tlie  left  the  beginning  of  an 
intermediate  layer;  B,  from  the  chin,  w'ith  three  well -developed  cjiidermal  layers. 

two  layers.  The  outer,  somewhat  flattened  cells  compose  the  transient 
periderm;  the  hasal  cells,  of  cuboidal  or  low  columnar  shape,  are  the  repro- 
ducing elements  which  gradually  give  rise  to  new  strata  above  (Fig.  229  A). 

During  the  third  and  fourth  months,  the  epidermis  is  typically  three- 
layered, an  intermediate  stratum  being  interposed  between  the  basal  and 
periderm  cells  (Fig.  220  /?).  After  the  fourth  month,  the  epidermis  becomes 


THE  NAILS 


highly  stratified.  The  inner  layers  of  actively  dividing  cells  then  con- 
stitute the  definitive  stratum  germinativum.  The  outer  layers  cornify 
progressively  toward  the  surface.  Thus,  above  the  germinative  cells  is 
the  thin  stratum  granulosiim,  containing  keratohyalin  granules.  Next 
higher,  lies  the  clear  stratum  liicidum  whose  fluid,  eleidin  content  is  supposed 
to  represent  softened  keratohyalin.  Still  nearer  the  surface,  the  thickened 
ectoplasm  becomes  cornifled,  and  in  the  cytoplasm  a fatty  substance  col- 
lects; these  gradually  flattened  cells  comprise  the  stratum  corneiim. 

When  the  hairs  emerge,  at  about  the  sixth  month,  they  do  not  pene- 
trate the  outer  periderm  of  the  epidermis,  but  lift  it  off.  Hence,  in 
mammals,  this  layer  is  known  also  as  the  epitrichiiim  (layer  upon  the  hair). 
Desquamated  epitrichial  and  epidermal  cells  mingle  with  cast-off  lanugo 
hairs  and  sebaceous  secretions  to  form  the  pasty  vernix  caseosa  that  smears 
the  fetal  skin.  Pigment  granules  appear  soon  after  birth  in  the  cells  of 
the  stratum  germinativum;  these  granules  are  probably  elaborated  by  the 
cytoplasm.  Negro  infants  are  quite  light  in  color  at  birth,  but  within 
six  weeks  their  integument  reaches  the  final  degree  of  pigmentation. 

The  Derma  or  Corium. — -The  origin  of  the  fibrous  layers  of  the  chick’s 
integument  may  be  traced  to  that  lateral  portion  of  a mesodermal  segment 
termed  the  cutis  plate  or  dermatome  (Fig.  212).  It  is  now  claimed  that 
mammals  lack  a dermatome  and  that  the  region  usually  so  designated 
really  is  a part  of  the  myotome.  In  this  event,  the  connective-tissue  cor- 
ium differentiates  directly  from  the  mesenchyme  subjacent  to  the  epider- 
mis. At  about  the  end  of  the  third  month,  a distinction  between  the 
compact  corium  proper  and  the  looser  subcutaneous  tissue  becomes 
recognizable.  From  the  corium,  papillae  project  into  the  germinative 
stratum. 

Anomalies. — The  deposition  of  pigment  in  the  epidermis  and  elsewhere  may  fail 
{albinism),  or  be  over-abundant  {melanism).  The  defects  of  pigmentation  sometimes  af- 
fect local  areas  only.  Navi  are  either  pigmented  spots  Cmoles’),  or  purple  discolorations 
{'birthmarks’)  caused  by  cavernous  vascular  plexuses  in  the  corium.  Ichthyosis  represents 
an  excessive  thickening  of  the  stratum  corneuni.  In  severe  cases,  horny  plates,  separated 
by  deep  cracks,  are  formed.  Dermoid  cysts  (p.  156),  resulting  from  epidermal  inclusions, 
are  not  infrequent  along  the  lines  of  fusion  of  embryonic  structures  (e.g.,  branchial  grooves, 
mid-dorsal  and  mid-ventral  body  wall). 

THE  NAILS 

Nails  are  modifications  of  the  epidermis  that  correspond  to  the  claws 
and  hoofs  of  lower  mammals.  The  nail  anlage  is  recognizable  in  fetuses 
of  10  weeks  as  an  epidermal,  pouch-like  fold  that  soon  extends  from  the 
proximal  border  of  the  future  exposed  plate  almost  to  the  articulation  of 
the  terminal  phalanx  (Fig.  230  C)  \ this  proximal  nail  fold  also  continues 
laterally  on  either  side  as  the  lateral  nail  folds  (Fig.  230  A,  B). 


2^2 


TJIE  INTEGUMENTARY  SYSTEM 


The  material  of  the  nail  is  developed  in  the  lower  lamina  of  the 
]:>roximal  nail  fold  (Tig.  230  C).  Certain  of  the  epidermal  cells,  which, 
according  to  Bowen,  represent  a modified  stratum  lucidum,  develop 
keratin  fil)rils  during  the  fifth  fetal  month.  These  appear  without  the 
preliminary  keratohyalin  stage,  as  is  the  case  in  ordinary  epidermis. 
The  cells  flatten  and  form  the  compact  mass  of  which  the  nail  plate  is 
composed.  Thus,  the  nail  substance  differentiates  in  the  proximal  nail 
fold  as  far  distad  as  the  outer  edge  of  the  Imuila  (the  whitish  crescent  at  the 
base  of  the  adult  nail).  Beyond  the  lunule,  the  underlying  epidermis  takes 


A B 


Fk;.  230. — The  develo])ment  of  the  human  nail  (Kollman).  .1,  10  weeks  (X  20);  B,  14  weeks 
(X  1,3);  C,  longitudinal  section  at  14  weeks  (X  24). 

no  active  part  in  development.  The  stratum  corneum  and  periderm  of 
the  epidermis  for  a time  cover  completely  the  free  nail  and  are  termed  the 
eponychium  (Fig.  230  C).  In  late  fetuses  this  is  lost,  but  portions^  of 
the  horny  layer  persist  as  the  curved  rim  of  epidermis  which  adheres  to 
the  base  of  the  adult  nail.  During  life  the  nail  constantly  grows  at  its 
base  (proximally),  is  shifted  distally  over  the  nail  bed,  and  projects  at  the 
tip  of  the  digit.  The  coriuni  throws  its  surface  of  contact  with  the  nail 
into  parallel  longitudinal  folds  that  produce  the  characteristic  ridging. 

THE  HAIR 

Hairs  are  specialized  epidermal  gowths.  The  earliest  begin  to  develop 
at  the  end  of  the  second  month  on  the  eyebrows,  upper  lip,  and  chin;  those 
of  the  general  integument  appear  at  the  beginning  of  the  fourth  month. 

The  first  evidence  of  a hair  anlage  is  the  crowding  and  elongation  of  a 
cluster  of  germinative  cells  (Fig.  231,  A).  Their  bases  sink  bud-like  into 


THE  H.A.IR 


233 


the  corium,  and  active  proliferation  soon  produces  a cylindrical  epithelial 
plug  (Fig.  23  ] , 5,  C).  This  hair  anlage  consists  of  an  outer  wall  of  colum- 


Ccnlral  cells 

Epidermal  anlage  of 
hair  B 

Anlage  of  hair  papilla 


Epitrichium 


Epidermal  anlage 
hair  A 


Anlage  of  hair  papilla 


Epidermal  anlage  of  hair  C 


Mesefichymal 
Hair 
Hair  papilla' 


Fig.  231. — Section  through  the  facial  integument  of  a three-months’  fetus,  showing  three 
stages  in  the  early  development  of  hair  (Prentiss).  X 330. 


Fig.  232. — Longitudinal  section  through  a developing  hair  from  a five  and  one-half  months’ 

fetus  (Stdhr  in  Prentiss).  X 220. 

nar  cells,  continuous  with  the  basal  layer  of  the  epidermis,  and  an  internal 
mass  of  polyhedral  cells.  About  the  whole  is  a mesenchymal  sheath,  and 
at  its  base  the  mesenchyme  condenses  into  a mound-like  papilla. 


2,34 


THE  INTEGUMENTARY  SYSTEM 


As  (leveloi)ment  proceeds  and  the  hair  anlage  pushes  deeper  into 
the  corium,  its  base  enlarges  into  the  bulb  which  becomes  moulded  over  the 
papilla  (Fig.  232).  The  actual  hair  substance  is  a proliferation  from  the 
basal  epidermal  cells  next  the  papilla.  These  cells  give  rise  to  an  axial 
core,  destined  to  become  the  inner  sheath  and  shaft,  which  grows  toward  the 
surface  (Fig.  232).  Entirely  distinct  are  the  peripheral  cells  of  the  original 
anlage  which  constitute  the  outer  sheath. 

The  hair  grows  at  its  base  and  is  pushed  up  through  the  eentral 
cells  of  the  primordial  downgrowth.  Above  the  level  of  the  bulb,  the  eells 
of  the  hair  shaft  cornify  and  differentiate  into  an  outer  cuticle,  middle 
cortex,  and  central,  inconstant  medulla.  Two  swellings  of  the  outer  hair 
sheath  appear  on  the  lower  side  of  the  obliquely  directed  follicle.  (Fig.  232). 
The  ujiper  of  these  becomes  the  associated  sebaceous  gland;  the  deeper 
swelling  is  the  epithelial  bed,  a region  of  rapid  mitosis  that  contributes  to 
the  growth  of  the  hair  follicle.  Mesenchymal  tissue  near  the  epithelial 
bed  transforms  into  the  smooth  fibers  of  the  arrcctor  pili  muscle.  Pigment 
granules  develop  in  the  basal  cells  of  the  hair  and  cause  its  characteristic 
coloration. 

The  first  generation  of  dense,  silky  ‘ lanugo  ’ hairs  are  short-lived,  all 
except  those  covering  the  face  being  cast  off  soon  after  birth.  The  coarser, 
replacing  hairs  develop,  at  least  in  part,  from  new  follicles.  Thereafter, 
hair  is  shed  and  regenerated  periodically  throughout  life. 

Anomalies.  --  Hypertrichosis  refers  to  excessive  hairiness  which  may  be  general  or  local, 
as  in  exhibited  ‘ hairy  monsters.’  In  the  rare  hypotrichosis,  the  congenital  absence  of  hair 
is  usually  associated  with  defective  teeth  and  nails. 

SEBACEOUS  GLANDS 

Nine-tenths  of  all  sebaceous  glands  accompany  hairs  but  independent 
ones  occur  also,  such  as  those  around  the  nostrils,  anus,  and  eyelids.  They 
appear  first  in  the  fifth  month  as  solid  buds  of  the  epidermis,  especially 
that  of  the  hair  follicles  (Fig.  232).  The  anlage  becomes  a lobulated  sac. 
A lumen  forms  by  the  fatty  degeneration  of  the  central  cells,  and  the 
resultant  oih"  secretion  is  an  important  constitutent  of  vernix  caseosa 
(p.  231). 

SWEAT  GLANDS 

Sudoriferous  glands  begin  to  develop  in  the  fourth  month  from  the 
epidermis  of  the  finger  tips,  the  palms  of  the  hands,  and  the  soles  of  the  feet. 
They  are  formed  as  solid  downgrowths  from  the  epidermis,  but  differ 
from  hair  anlages  in  being  more  compact  and  in  lacking  the  mesenchymal 
papillie  at  their  bases  (Fig.  233,  A,  B).  During  the  sixth  month  the  simple 
cords  coil,  and,  in  the  seventh  month,  their  lumina  appear  (Fig.  233,  C,  D). 
The  inner  layer  of  eells  forms  the  gland  cells,  while  the  outer  cells  become 


MAMMARY  GLANDS 


235 


transformed  into  smooth  muscle  fibers,  which,  accordingly,  are  ectodermal. 
In  the  axilla,  eyelids,  and  external  acoustic  meatus  the  sweat  glands  are 
large  and  branched. 

C D 


A B 


Fig.  233. — Vertical  sections  of  the  integument,  illustrating  four  stages  in  the  development 
of  a sweat  gland  (adapted  from  Kollman).  A,  B,  Four  months;  C,  D,  seven  months. 

MAMMARY  GLANDS 

Mammary  glands  are  peculiar  to  mammals.  In  embryos  of  about  9 
mm.  paired  ectodermal  thickenings  extend  lengthwise  between  the  bases 
of  the  limb  buds  (Fig.  234).  This  linear  ridge  is  the  milk  line.  In  man  it 


A mpulated  limb  bud 

Position  of  definitive 
gland  anlage 

ilk  line 


Fig.  234. — Human  embryo  of  13.5  mm.  with  a prominent  milk  line  (after  Kollman).  X 5. 

often  is  inconspicuous  except  in  the  pectoral  region,  but  in  lower  mammals, 
like  the  pig,  that  have  serially  repeated  glands,  a prominent  thickening 
reaches  from  axilla  to  groin  (Fig.  389). 


236 


THE  INTEGUMENTARY  SYSTEM 


Each  human  mammary  gland  begins  as  a thickening  and  downgrowth 
from  the  epidermal  milk  line  in  the  region  of  the  future  breast  (Fig.  235  ^)- 
During  the  fifth  month,  from  15  to  20  solid  cords  bud  off  {B).  These 
anlages  of  the  milk  ducts  branch  in  the  mesenchymal  tissue  of  the  corium, 
hollow  out,  and  eventually  produce  the  alveolar  oid  pieces  (C).  Where 
the  milk  ducts  open  on  the  surface  the  epidermis  is  elevated  to  form 
the  nipple,  but  this  may  be  delayed  until  after  birth.  The  glands  yield  a 
little  secretion  (‘witch  milk’)  at  birth;  they  enlarge  rapidly  at  puberty 
and  are  further  augmented  during  pregnancy,  while  two  or  three  days 
after  parturition  they  become  functionally  active. 

The  mammary  glands  are  regarded  by  most  authorities  as  modified 
sweat  glands.  This  homology  is  made  because  their  development  is 


Gl.uid  (Ullage  Epidermis 


N i p pie 


Fig.  235. — Sections  rei>rcsenting  three  stages  in  the  development  of  the  human  mammary 
gland  (Tourneux).  A,  Two  months;  B,  four  months;  C,  seven  months. 

similar,  and  because  in  the  lower  mammals  their  structure  is  the  same. 
Rudimentary  mammary  glands  (areolar  glands  of  Montgomery),  which 
also  resemble  sweat  glands,  occur  in  the  areola  about  the  nipple.  In 
many  mammals,  numerous  pairs  of  mammary  glands  are  developed  along 
the  milk  line  (pig;  dog) ; in  some  a single  pair  occurs  in  the  pectoral  region 
(primates;  elephant);  in  others,  they  are  confined  to  the  inguinal  region 
(sheep;  cow;  horse). 

Anomalies.- -Supernumerary  mammary  glands  (hypermaslia)  or  nipples  (liy per- 
ihelia) between  the  axilla  and  groin  are  common.  These  represent  independent  differen- 
tiations along  the  primitive  milk  line,  such  as  occur  normally  in  some  mammals. 


CHAPTER  XIII 


THE  CENTRAL  NERVOUS  SYSTEM 
I.  HISTOGENESIS  OF  THE  NERVOUS  TISSUES 


The  nervous  tissues  and  sensory  epithelia  are  derived  from  portions 
of  the  primitive  integument.  The  anlage  of  the  entire  nervous  system  is  a 
thickened  band  of  ectoderm  along  the  mid-dorsal  line  of  the  embryo. 
This  is  the  neural  plate  (Figs.  57  and  58),  which,  in  embryos  of  2 mm., 
develops  a deep  neural  groove,  bounded  laterally  by  paired  neural  folds 
(Fig.  236  A-C).  The  folds  presently  meet  and  fuse,  thereby  forming  the 


Ectoderm 


Nc’iral  groove 


-Neural  fold 


Fig.  236. — Sections  of  the  developing  neural  tube  in  human  embryos  (adapted  by  Prentiss). 
A,  An  early  stage;  B,  2 mm.;  C,  2 mm.;  D,  2.7  mm. 


neural  tube  (D)  which  lies  below  the  surface  of  the  general  ectoderm  and 
becomes  separated  from  it  (Fig.  241).  The  cells  of  this  tube,  and  its 
associtaed  ganglion  crests,  give  rise  to  all  the  nervous  tissues,  with  the 
single  exception  of  the  nerve  cells  and  fibers  of  the  olfactory  epithelium. 

The  cells  of  the  neural  tube  differentiate  into  two  products.  These 
are  nerve  cells,  in  which  irritability  and  conductivity  have  become  the 
predominant  functions,  and  neuroglia  cells,  which  constitute  the  distinc- 
tive supporting  tissue  of  the  nervous  system.  The  wall  of  the  neural  tube, 
consisting  at  first  of  a single  layer  of  columnar  cells  (Fig.  237  A),  becomes 

237 


238 


THE  CENTRAL  NERVOUS  SYSTEM 


many-layered;  the  com]jonent  cells  lose  their  sharp  outlines  and  form  a 
comijact  syncytium  which  is  bounded,  on  its  outer  and  inner  surfaces, 
by  an  external  and  lutcrual  limiting  membrane  (B,  C ).  In  lo  mm.  embrvos, 
the  cellular  strands  of  the  syncytium  are  arranged  radially  and  nearly 
parallel  (D) . The  nuclei  are  now  so  grouped  that  there  may  be  distinguished 


Marginal  layer  Mantle  layer  Ependymal  layer 

^ ! -TX-  xr' 


Germinal 

cell 


Marginal  layer  Ependymal  layer 


Internal  limiting  membrane 


Mesoderm  Marginal  layer 


Ependymal  layer 


'Germinal 

eell 


External  limiting  membrane  Mantle  layer  Internal  limiting  membrane 


D 


External  limiting  membrane 


Germinal  cell  Internal  limiting  membrane 


Mesoderm  Marginal  layer  * Mantle  layer  Ependymal  layer 


Fig.  237. — The  differentiation  of  the  neural  tube  (Hardesty).  X 690.  .4,  From  a rabbit 

embryo  before  the  closure  of  the  neural  tube;  B,  from  a 5 mm.  pig  embryo  after  closure;  C, 
from  a 7 mm.  pig  embryo;  /t,  from  a 10  mm.  pig  embryo.  *,  Boundary  between  mantle  and 
marginal  layers. 


three  layers:  (i)  an  inner  ependymal  zone,  with  cells  abutting  on  the 
internal  limiting  membrane  and  their  processes  extending  peripherally; 
(2)  a middle,  nucleated  mantle  zone;  and  (3)  an  outer,  non-cellular  marginal 
zone,  into  which  nerve  fibers  grow.  The  ependymal  zone  contributes  cells 
for  the  development  of  the  mantle  layer  (A~D).  The  cellular  mantle 


HISTOGENESIS  OF  THE  NERVOUS  TISSUES 


239 


layer  forms  the  gray  substance  of  the  central  nervous  system,  while  the 
fibrous  marginal  layer  constitutes  the  white  substance. 

The  primitive  germinal  cells  of  the  neural  tube  divide  by  mitosis 
and  give  rise  to  the  ependymal  cells  of  the  ependymal  zone  and  to  indifferent 
cells  of  the  mantle  layer  (Fig.  238).  From  these  latter  differentiate 
spongioblasts  and  neiiroblasts.  The  spongioblasts  transform  into  neuroglia 
cells  Sind  fibers,  which  become  the  supporting  tissue  of  the  central  nervous 
system ; the  neuroblasts  are  primitive  nerve  cells,  which  develop  cell 
processes  and  are  converted  into  neurons.  A neuron  is  the  structural  and 
functional  unit  of  nervous  tissue. 

The  Differentiation  of  Neuroblasts. — ^The  nerve  fibers  develop  as 
outgrowths  from  the  neuroblasts,  and  a nerve  cell  with  all  its  processes 

.1  B 


Fig.  238. — Diagrams  showing  the  differentiation  of  the  cells  of  the  neural  tube  (after  Schaper). 

constitutes  a neuron.  The  origin  of  the  nerve  fibers  as  processes  of  the 
neuroblasts  is  seen  best  in  the  development  of  the  root  fibers  of  the  spinal 
nerves. 

The  Development  of  Efferent  Neurons. — ^At  the  end  of  the  first  month, 
clusters  of  neuroblasts  separate  from  the  general  syncytium  in  the  mantle 
layer  of  the  neural  tube.  The  neuroblasts  become  pear-shaped,  and  from 
the  small  end  of  the  cell  a slender,  primary  process  grows  out  (Fig.  239  A, 
‘F'\  B).  This  process  is  the  axon,  or  axis  cylinder.  Such  primary  proc- 
esses may  course  in  the  marginal  layer  of  the  neural  tube  (Fig.  239  A, 
a),  or,  converging,  may  penetrate  the  marginal  layer  ventro-laterally 
and  form  the  ventral  roots  of  the  spinal  nerves  (Fig.  240).  Similarly, 
the  efferent  fibers  of  the  cerebral  nerves  grow  out  from  neuroblasts  of  the 
brain  wall.  Within  the  cytoplasm  of  young  nerve  cells  and  their  primary 
processes,  strands  of  fine  fibrils  occur  (Fig.  239  A).  These,  the  ncurofibrillcc, 
are  usually  assumed  to  be  the  conducting  elements  of  the  neurons.  The 
cell  bodies  of  the  efferent  neurons  soon  become  multipolar  by  the  develop- 
ment of  branched  secondary  processes,  the  dendrons  or  dendrites. 


240 


THE  CENTRAL  NERVOUS  SYSTEM 


Fig.  239. — The  differentiation  of  neiiroblasts  in  chick  embryos  of  the  third  day  (Cajal). 
A,  Transverse  section  through  the  spinal  cord,  showing  axons  (F)  growing  from  neuroblasts 
into  the  ventral  root,  and  from  bipolar  ganglion  cells  (i/)  into  the  dorsal  root.  B Single 
neuroblasts,  showing  neurofibrils  and  growing  tip(*). 


Fig.  240. — Transverse  section  of  the  spinal  cord  from  a human  embryo  of  five  weeks,  showing 
the  origin  of  ventral  root  fibers  from  neuroblasts  (His).  X 130. 


HISTOGENESIS  OF  THE  NERVOUS  TISSUES 


241 


Development  of  the  Spinal  Ganglia  and  Afferent  Neurons. — After  the 
formation  of  the  neural  plate  and  groove,  a longitudinal  ridge  of  cells 
appears  on  each  side  where  the  ectoderm  and  neural  plate  join  (Fig.  241  A). 
This  ridge  of  ectodermal  cells  is  the  neural,  or  ganglion  crest.  When  the 
neural  tube  closes  and  the  ectoderm  separates  from  it,  the  cells  of  the 
ganglion  crest  overlie  the  neural  tube  dorso-laterally  (Fig.  241  B,  C).  As 
development  continues  they  separate  into  right  and  left  linear  crests, 
distinct  from  the  neural  tube,  and  migrate  ventro-laterally  to  a position 


Neural  crest 


Fig.  241. — The  development  of  the  ganglion  crest  in  a 2.5  mm.  human  embryo  (after 

Lenhossek). 

between  the  neural  tube  and  myotomes  (Fig.  212).  In  this  position,  the 
ganglion  crest  forms  a band  of  cells  extending  the  whole  length  of  the 
spinal  cord  and  as  far  cephalad  as  the  otic  vesicles.  At  regular  intervals 
in  its  course  along  the  spinal  cord,  the  proliferating  cells  of  the  crest  give_-,i.  - /,■ , 

ri^se  to  enlargements,  the  spinal  ganglia  (Figs.  278  and  279).  The  spinal  a -/V  - 
ganglia  are  arranged  segmentally  and  are  connected  at  first  by  cellular 
bridges  that  later  disappear.  In  the  hind-brain  region,  certain  ganglia  • 

of  the  cranial  nerves  develop  also  from  the  crest  but  are  not  segmentally 
arranged. 

The  cells  of  the  spinal  ganglia  differentiate  into  ganglion  cells  and 
supporting  cells,  groups  which  are  comparable  to  the  neuroblasts  and 
spongioblasts  of  the  neural  tube.  The  neuroblasts  of  the  ganglia  become 
fusiform  and  develop  a primary  process  at  each  pole;  thus,  these  neurons 

16 


/>•  -j-f  . 

C uW  a.  No  fo  ^ I 


C,  7-  ^£^2  ^ y. 


2_12 


THE  CENTRAL  NERVOUS  SYSTEM 


are  of  the  bipolar  type  (Fig.  23Q  A,  d).  The  centrally  directed  processes 
of  the  ganglion  cells  converge,  and,  by  elongation,  form  the  dorsal  roots. 
They  penetrate  the  dorso-lateral  wall  of  the  neural  tube,  bifurcate,  and 
course  cranially  and  caudally  in  the  marginal  layer  of  the  spinal  cord. 
By  means  of  branched  processes  they  come  in  contact  with  the  neurons 
of  the  mantle  layer.  The  peripheral  processes  of  the  ganglion  cells, 
as  the  dorsal  spinal  roots,  join  the  ventral  roots,  and  with  them  constitute 
the  trunks  of  the  spinal  nerves  (Fig.  246). 

At  first  bijjolar  (Fig.  242,  A),  the  majority  of  the  ganglion  cells 
become  unipolar,  either  by  the  fusion  of  the  two  primary  processes  or  by 


Fig.  242. — Stages  in  the  formation  of  unipolar  ganglion  cells  (Cajal).  From  a human  fetus  of 

ten  weeks. 


the  bifurcation  of  a single  process  (C).  The  process  of  the  unipolar 
ganglion  is  then  T-sha])ed  (B)-  Many  of  the  bipolar  ganglion  cells 
persist  in  the  adult,  while  others  develop  several  secondary  processes 
and  thus  become  multipolar  in  form.  In  addition  to  forming  the  spinal 
and  cranial  ganglion  cells,  neuroblasts  of  the  ganglion  crest  are  believed 
to  migrate  ventrally  and  form  the  sympathetic  ganglia  (Fig.  246). 

Differentiation  of  the  Supporting  Elements.  In  the  Neural  Tube. — ■ 
The  si)ongioblasts  of  the  neural  tube  differentiate  into  the  supporting 
tissue  of  the  central  nervous  system.  This  includes  the  ependymal  cells, 
which  line  the  neural  cavity  and  constitute  one  of  the  primary  layers  of 
the  neural  tube,  and  neuroglia  cells  and  their  fibers. 


HISTOGENESIS  OF  THE  NERVOUS  TISSUES 


243 


A preceding  paragraph  describes  how  the  strands  of  the  syncytium, 
formed  by  the  spongioblasts,  become  arranged  radially  in  the  neural  tube 
of  early  embryos  (Fig.  237  D).  As  the  wall  thickens,  the  strands  elongate 
equally  and  form  a radiating,  branched  framework  (Fig.  243).  The  group 
of  spongioblasts  which  lines  the  neural  cavity  constitutes  the  ependymal 
layer.  Processes  from  these  cells  extend  through  the  neural  tube,  even 
to  its  periphery.  The  cell  bodies  are  columnar  and  persist  as  the  lining 
of  the  central  cavities  of  the  spinal  cord  and  brain  (Fig.  244). 

Near  the  midplane  of  the  adult  spinal  cord,  both  dorsally  and  ven- 
trally,  the  supporting  tissue  retains  its  primitive  ependymal  structure 


B 


Fig.  243. — Ependymal  cells  from  the  neural  tube  of  a chick  (Cajal).  .4,  Embryo  of  first  day; 

B,  of  third  day. 


(Fig.  244).  Elsewhere,  the  supporting  framework  is  differentiated  into 
neuroglia  cells  and  fibers.  The  neuroglia  cells  form  part  of  the  spongio- 
blastic  syncytium  and  are  scattered  through  the  mantle  and  marginal 
layers  of  the  neural  tube.  By  proliferation  they  increase  in  number,  and 
their  form  depends  upon  the  pressure  of  the  nerve  cells  and  fibers  which 
develop  around  them.  Neuroglia  fibers  are  differentiated  in  a manner 
comparable  to  that  of  connective-tissue  fibers  (Fig.  203).  As  the  cyto- 
plasmic processes  of  the  neuroglia  cells  primarily  form  a syncytium,  the 
fibers  may  extend  from  cell  to  cell.  The  neuroglia  fibers  develop  late  in 
fetal  life  and  undergo  a chemical  transformation  into  neurokeratin,  the 
same  substance  that  is  found  in  the  sheaths  of  myelinated  fibers. 


244 


THE  CENTRAL  NERVOUS  SYSTEM 


Supporting  Elements  of  the  Ganglia. — The  supporting  cells  of  thespinal 
ganglia  at  first  form  a syncytium,  in  the  meshes  of  which  are  found  the 
neuroblasts.  They  differentiate  into  flattened  capsule  eells,  which  encap- 
sulate the  ganglion  cells,  and  into  sheath  cells,  which  envelop  the  axon 
processes  of  both  dorsal  and  ventral  root  fibers  and  are  continuous  with 
the  capsules  of  the  ganglion.  It  is  certain  that  many  of  the  sheath  cells 
migrate  peripherally  along  with  the  developing  nerve  fibers.  They  are 


Fig.  244. — Ependymal  cells  of  the  spinal  cord,  from  a fetus  of  ten  weeks  (Cajal).  /I, 
Floor  plate;  B,  central  canal;  C,  line  of  future  fusion  of  neural  walls;  E,  ependymal  cells;  *, 
neuroglia  cells  and  fibers. 


at  first  spindle-shaped,  and,  as  primary  sheaths,  enclose  bundles  of  nerve 
fibers.  Later,  by  the  proliferation  of  the  sheath  cells,  the  bundles  are 
separated  into  single  fibers,  each  with  its  sheath  of  Schwann,  or  neuri- 
lemma. Every  sheath  cell  forms  a segment  of  the  neurilemma,  the  limits 
of  contiguous  cells  being  indicated  by  constrictions,  the  nodes  of  Ranvier. 

The  Myelin  Sheath. — -During  the  fourth  month  an  inner  myelin,  or 
medullary  sheath  appears  about  many  nerve  fibers.  This  consists  of  a 
spongy  framework  of  neurokeratin  in  the  interstices  of  which  a fatty  sub- 
stance, myelin,  is  deposited.  The  origin  of  the  myelin  sheath  is  in  doubt. 


MORPHOGENESIS  OF  THE  CENTRAL  NERVOUS  SYSTEM 


245 


By  some  ( Ranvier)  it  is  believed  to  be  a differentiation  of  the  neurilemima, 
the  myelin  being  deposited  in  the  substance  of  the  nucleated  sheath  cell. 
Others  regard  the  myelin  as  a direct  product  of  the  axis  cylinder  (Kol- 
liker),  or  as  an  intercellular  substance  precipitated  through  its  influence 
(Bardeen).  The  integrity  of  myelin  is  dependent  at  least  upon  the  nerve 
cell  and  axis  cylinder,  for,  when  a nerve  is  cut,  it  very  soon  shows  degener- 
ative changes.  In  the  central  nervous  system  there  is  no  distinct  neuri- 
lemma sheath  investing  the  fibers.  However,  sheath  cells  are  said  to  be 
present  and  most  numerous  during  the  period  when  myelin  is  developed. 
Hardesty  traces  their  origin  to  the  spongioblastic  supporting  cells  of  the 
neural  tube,  and  believes  that  the  myelin  of  the  fibers  arises  in  the 
interspaces. 

The  myelinated  fibers,  those  with  a myelin  sheath,  have  a glistening 
white  appearance  and  give  the  characteristic  color  to  the  white  substance 
of  the  central  nervous  system  and  to  the  peripheral  nerves.  The  fibers 
which  are  first  functional  receive  their  myelin  sheaths  first.  This  process 
is  only  completed  during  the  third  year  of  infancy.  Many  of  the  periph- 
eral fibers,  especially  those  of  the  sympathetic  system,  remain  unmye- 
linated but  are  supplied  with  a neurilemma  sheath.  Large  numbers  of 
unmyelinated  fibers  occur  also  in  the  peripheral  nerves  and  spinal  cord. 

The  Neuron  Doctrine. — The  neuron  concept  of  the  development  of  nerve  fibers  is  the 
one  generally  adopted  at  the  present  time.  It  assumes  that  all  axons  and  dendrites  are 
formed  as  outgrowths  from  nerve  cells,  an  hypothesis  first  promulgated  by  His.  The 
embryological  evidence  is  supported  by  experiment.  It  has  long  been  known  from  the 
work  of  Waller  that  if  nerves  are  severed,  the  fibers  distal  to  the  point  of  section,  and 
thus  isolated  from  their  nerve  cells,  will  degenerate,  but  that  regeneration  will  take  place 
from  the  central  stumps  of  cut  nerves,  the  fibers  of  which  are  still  connected  with  their 
cells.  IMore  recently,  Harrison,  experimenting  on  amphibian  larvse,  has  shown  that 
peripheral  nerves  do  not  develop  if  the  neural  tube  and  crest  are  removed,  and  that 
isolated  ganglion  cells  growing  in  clotted  lymph  will  give  rise  to  long  axon  processes  in 
the  course  of  four  or  five  hours. 

A second  theory,  supported  by  Schwann,  Balfour,  Dohm,  and  Bethe,  but  not  widely 
credited,  assumes  that  the  nerve  fibers  are  in  part  differentiated  from  a chain  of  cells,  so 
that  the  neuron  would  represent  a multicellular,  not  a unicellular  structure.  Apathy  and 
0.  Schulze  modified  this  cell-chain  theory  by  assuming  that  the  nerve  fibers  differentiate  in  a 
syncytium  which  intervenes  between  the  neural  tube  and  the  peripheral  end  organs.  Held 
further  modified  the  theory  by  claiming  that  the  proximal  portion  of  a nerve  fiber  is 
derived  from  the  neuroblast  or  ganglion  cell  and  that  this  grows  into  a syncj-ffium  which 
gives  rise  to  the  peripheral  portion . 

II.  Morphogenesis  of  the  Central  Nervous  System 

The  primitive  neural  tube  is  formed  by  the  folding  of  the  neural  plate 
into  an  epithelial  tube,  as  described  in  the  previous  section.  The  groove 
begins  to  close  in  embryos  of  2 mm.  along  the  mid-dorsal  line,  near  the 
middle  of  the  body,  and  the  closure  advances  in  both  directions  (Fig.  245). 


246 


THE  CENTRAL  NERVOUS  SYSTEM 


Until  after  the  fourth  week,  however,  there  still  persists  a neuroporic 
opening  at  each  end  of  the  neural  tube,  somewhat  dorsad  (Fig.  251). 
But  before  the  closure  of  the  neuropores,  even  in  embryos  of  2.5  mm.  or 
less,  the  cranial  end  of  the  neural  tube  has  enlarged  and  constricted  at 
two  points  to  form  the  three  primary  brain  vesicles.  The  caudal  two-thirds 


Fig.  245. — Human  embryo  of  2.4  mm.,  with  a ])artially  closed  neural  tube  (Kollmanni.  X 30. 

of  the  neural  tube,  which  remains  smaller  in  diameter,  is  the  anlage  of  the 
spinal  cord. 

THE  SPINAL  CORD 

The  spinal  portion  of  the  neural  tube  is  at  first  nearly  straight,  but  as 
the  embryo  flexes  it  also  is  bent  into  a curve,  convex  dorsally  (Fig.  282). 
Its  wall  gradually  thickens  during  the  first  month  and  the  diameter  of  the 
central  canal  is  diminished  from  side  to  side.  By  the  end  of  the  first  month, 
three  layers  have  been  developed  in  the  manner  already  described  (Fig. 
246).  These  layers  are  the  inner  ependymal  layer,  which  forms  a narrow 
zone  about  the  neural  cavity,  the  middle,  cellular  mantle  layer,  and  the 
outer,  fibrous  marginal  layer. 

The  Ependymal  Layer  differentiates  into  a dorsal  roof  plate  and  a 
ventral  floor  plate  (Fig.  247).  Laterally,  its  proliferating  cells  contribute 


THE  SPINAL  CORD 


247 


Mantle  layer 


Dorsal  ramus 


Ventral 

root 


Marginal  layer 


Dorsal 
Ependymal  layer 
Spinal 


Neural  cavity 


Nerve  trunk  Sympathetic  ganglion 

Fig.  246. — Transverse  section  through  a 10  mm.  human  embryo  at  the  level  of  the  arm  buds- 
showing  the  spinal  cord  and  a spinal  nerve  (Prentiss).  X 44. 


X 44- 


248 


THE  CENTRAL  NERVOUS  SYSTEM 


neuroblasts  and  neuroglia  cells  to  the  mantle  layer.  This  proliferation 
ceases  first  in  the  ventral  floor,  which  is  thus  narrower  than  the  dorsal 
portion  in  10  to  20  mm.  embryos  (Figs.  246  and  247).  The  neural  cavity 
is  for  a time  somewhat  rhomboidal  in  transverse  section,  wider  dorsally 
than  ventrally.  Its  lateral  angle  forms  the  sulcus  limitans  (Fig.  255), 
which  marks  the  subdivision  of  the  lateral  walls  of  the  neural  tube  into 
the  donsal  alar  plate  (sensory)  and  ventral  basal  plate  (motor).  When  the 
ependymal  layer  ceases  to  contribute  new  cells  to  the  mantle  layer,  its 
walls  are  a])proximated  dorsally  (Fig.  247).  At  about  nine  weeks, 


Fi<;.  248. — Transverse  section  of  the  s])inal  cord  and  ganglion  from  a fetus  of  nine  weeks 

(Prentiss).  X 44. 

these  walls  fuse  and  the  dorsal  portion  of  the  neural  cavity  is  obliterated 
(Fig.  248);  in  a fetus  of  three  months,  the  persisting  cavity  is  becoming 
rounded  into  the  definite  central  canal  (Fig.  249).  The  cells  lining  the 
central  canal  are  ependymal  cells  proper.  Those  in  the  floor  of  the  canal 
form  the  persistent  floor  plate.  Their  fibers  extend  ventrad,  reaching  the 
surface  of  the  cord  in  the  depression  of  the  ventral  median  fissure. 

When  the  right  and  left  walls  of  the  ependymal  layer  fuse,  the  epen- 
dymal cells  of  the  roof  plate  no  longer  radiate,  but  form  a median  septum 
(Fig.  248).  Later,  as  the  marginal  layers  of  either  side  thicken  and  are 
approximated,  the  median  septum  is  extended  dorsally.  Thus,  the  roof 
plate  is  converted  into  part  of  the  dorsal  median  septum  of  the  adult  spinal 
cord  (Fig.  249). 

The  Mantle  Layer,  as  already  described,  receives  contributions  from 
the  proliferating  cells  of  the  ependymal  layer.  A ventro-lateral  thickening 
first  becomes  prominent  in  embryos  of  10  to  15  mm.  (Fig.  246).  This  is 
the  ventral  {anterior)  gray  coliinm,  which  in  later  stages  is  subdivided. 


THE  SPINAL  CORD 


249 


forming  also  a lateral  gray  colmnu  (Fig.  249).  It  is  a derivative  of  the  basal 
plate.  In  embryos  of  20  mm.,  a dorso-lateral  thickening  of  the  mantle 
layer  is  seen,  the  cells  of  which  constitute  the  dorsal  (posterior)  gray 
column  (Figs.  248  and  249);  about  these  cells  the  collaterals  of  the  dorsal 
root  fibers  end.  The  cells  of  the  dorsal  gray  column,  derivatives  of  the 
alar  plate  of  the  cord,  thus  form  terminal  nuclei  for  the  afferent  spinal 
nerve  fibers.  Dorsal  and  ventral  to  the  central  canal,  the  mantle  layer 
forms  the  dorsal  and  ventral  gray  commissures.  In  the  ventral  floor 
plate,  nerve  fibers  cross  from  both  sides  of  the  cord  as  the  ventral  white 
commisstire. 


Dorsal  median  septum  Fasciculus  gracilis 


Fig.  249. — Transverse  section  of  the  spinal  cord  from  a fetus  of  three  months  (Prentiss). 

X 44- 


The  Marginal  Layer  is  composed  primarily  of  a framework  of  neuroglia- 
and  ependymal-cell  processes.  Into  this  framework  grow  the  axons  of 
nerve  cells,  so  that  the  thickening  of  the  marginal  layer  is  due  to  the 
increasing  number  of  nerve  fibers  contributed  to  it  by  extrinsic  ganglion 
cells  and  neuroblasts.  When  their  myelin  develops,  these  fibers  form  the 
white  substance  of  the  spinal  cord. 

The  fibers  have  three  sources  (Fig.  281) ; (i)  they  may  arise  from  the  spinal  ganglion 
cells,  entering  as  dorsal  root  fibers  and  counsing  cranially  and  caudally  in  the  marginal 
layer;  (2)  they  may  arise  from  neuroblasts  in  the  mantle  layer  of  the  spinal  cord,  (a)  as 
fibers  which  connect  adjacent  nuclei  of  the  cord  (fasiculi  proprii  or  ground  bundles),  or  (b) 
as  fibers  which  extend  upward  to  the  brain;  (3)  they  may  arise  from  neuroblasts  of  the 
brain,  (a)  as  descending  tracts  from  the  brain  stem,  or  (6)  as  long,  descending  cortico- 
spinal tracts  from  the  cortex  of  the  cerebrum.  Of  these  fiber  tracts,  (i)  and  (2  a)  appear 
during  the  first  month;  (2  b)  and  (3  a)  dui'ing  the  third  month;  (3  b)  at  the  end  of  the  fifth 
month. 


THE  CENTRAL  NERVOUS  SYSTEM 


250 


The  dorsal  root  fibers  from  the  spinal  ganglion  cells,  entering  the  cord  dorso-laterally, 
subdivide  the  white  substance  of  the  marginal  layer  into  dorsal  and  lateral  funiculi  (Fig. 
248).  The  lateral  funiculus  is  marked  off  by  the  ventral  root  fibers  from  the  ventral 
funiculus  (Fig.  246).  The  ventral  root  fibers,  as  we  have  seen,  take  their  origin  from  the 
neuroblasts  of  the  ventral  gray  column  in  the  mantle  layer.  They  are  thus  derivatives  of 
the  basal  plate. 

The  dorsal  funiculus  is  formed  chiefly  by  the  dorsal  root  fibers  of  the  ganglion  cells, 
and  is  subdivided  into  two  distinct  bundles,  the  fasicul us  f^racilis,  median  in  position,  and 
the  fascicul us  nnicalus,  lateral  (Fig.  240).  The  dorsal  funiculi  are  separated  only  by  the 
dorsal  median  septum. 

The  lateral  and  ventral  funiculi  are  composed:  (i)  of  fasciculi  proprii,  or  ground  bun- 
dles, originating  in  the  spinal  cord;  (2)  of  ascending  tracts  from  the  cord  to  the  brain;  (3) 
of  the  descending  fiber  tracts  from  the  brain.  The  fibers  of  these  fasciculi  intermingle  and 
the  fasciculi  are  thus  without  sharp  boumlaries.  The  floor  plate  of  ependymal  cells  lags 
behind  in  its  development,  and,  as  it  is  interposed  between  the  thickening  right  and  left 
walls  of  the  ventral  funiculi,  these  do  not  meet  and  the  ventral  median  fissure  is  produced 
(cf.  Figs.  246  and  240). 

The  development  of  myelin  in  the  nerve  fibers  of  the  cord  begins  late  in  the  fourth 
month  of  fetal  life  and  is  completed  between  the  fifteenth  and  twentieth  years.  Myelin 
appears  first  in  the  root  fibers  of  the  spinal  nerves  and  in  those  of  the  ventral  commis.sure, 

next  in  the  ground  fjundles  and  dorsal  funiculi.  The  cortico- 
spinal (pyramidal)  fasciculi  are  the  tardiest;  they  become 
myelinated  during  the  first  and  second  years.  As  myelin 
appears  in  the  various  fiber  tracts  at  different  periods,  this 
condition  has  been  utilized  in  tracing  the  extent  and  origin 
of  the  various  fasciculi  in  the  central  nervous  system. 

The  Cervical  and  Lumbar  Enlargements. — 

The  spinal  cord  enlarges  at  the  levels  of  the  two 
nerve  plexuses  supplying  the  upper  and  lower 
extremities.  As  the  fibers  to  the  muscles  of  the 
extremities  arise  from  nerve  cells  in  the  ventral 
gray  column,  the  number  of  these  cells  and  the 
mass  of  the  gray  substance  is  increased ; since  larger 
numbers  of  fibers  from  the  integument  of  the 
limbs  also  enter  the  cord  at  this  level,  there  are 
likewise  present  more  cells  about  which  sensory 
Ihe  brain  and  cord  of  a three-  ^^ers  terminate.  Consequently,  there  is  formed 
months’ fetus  (Koiiiker).  at  the  level  of  the  origin  of  the  nerves  of  the 

brachial  plexus  the  cervical  enlargement,  and  oppo-’ 
site  the  origins  of  the  nerves  of  the  lumbo-sacral  plexus  the  lumbar 
enlargement  (Fig.  250). 

After  the  third  month,  the  vertebral  column  grows  faster  than  the 
spinal  cord.  Since  the  cord  is  anchored  to  the  brain,  the  vertebrae  and 
the  associated  roots  and  ganglia  of  the  spinal  nerves  shift  caudally  along  the 
cord.  For  this  reason,  the  origin  of  the  coccygeal  nerves  in  the  adult 
is  opposite  the  first  lumbar  vertebra  and  the  nerves  course  obliquely  down- 


THE  SPINAL  CORD 


251 

ward,  nearly  parallel  to  the  spinal  cord.  The  tip  of  the  neural  tubje  is 
attached  to  the  coccyx  during  this  period  of  unequal  growth,  so  its  caudal 
portion  becomes  stretched  into  the  slender,  solid  cord  known  as  the  filiim 
terminale.  The  obliquely  coursing  spinal  nerves,  with  the  filum  terminate, 
constitute  the  cauda  equina.  Traces  of  the  original  saccular  termination 
of  the  neural  tube  in  the  integument  are  recognizable  at  birth. 

THE  BRAIN 

Primary  Divisions. — The  neural  tube  in  embryos  of  2 to  2.5  mm.  is 
nearly  straight,  but  its  cranial  end  is  enlarged  to  form  the  anlage  of  the 
brain  (Fig.  245).  Three  regions  of  expansion,  separated  by  two  retarded 


Fig.  251. — Reconstructions  of  the  brain  of  a 3.2  mm.  human  embryo  (His-Prentiss). 
X about  35.  .4,  Lateral  surface;  B,  median  sagittal  section. 


zones  of  apparent  constriction,  subdivide  the  brain  into  three  primary  brain 
vesicles — the  fore-brain  (prosencephalon) , mid-brain  (mesencephalon) , 
and  hind-brain  (rhombencephalon). 

Both  the  fore-  and  hind-brain  vesicles  promptly  give  rise  to  two 
secondary  vesicles,  whereas  the  mid-brain  remains  undivided.  In 
embryos  of  about  3 mm.  (four  weeks),  the  fore-brain  shows  indication 
dorsally  of  a fold  which  subdivides  it  into  the  telencephalon,  with  its  primi- 
tive cerebral  hemispheres,  and  the  dicncephalon,  which  bears  the  optic 
vesicles  (Fig.  251).  The  mid-brain  retains  its  original  designation,  the 
mesencephalon . At  7 mm.  (five  weeks),  the  neuropores  have  closed  and 
the  hind-brain  constricts  into  the  mctencephalon , or  future  region  of  the 


252 


THE  CENTRAL  NERVOUS  SYSTEM 


cerebellum  and  pans,  and  into  the  myclencephalon,  or  medulla  oblongata. 
The  further  sej)aration  of  these  vesicles  may  be  followed  easily  in  stages 
of  lo  mm.  (six  weeks)  (Figs.  262  and  265),  and  14  mm.  (nearly  seven 
weeks)  (Fig.  253). 


PaUium 


Cephalic 

flexure 


Mctcn- 
ccphalon 
Corpus  striatum 
Optic  recess 

Hypothalamus 


Thalamus 


Isthmus 


Medulla  oblongata 


Myclencephalon 


Fig.  252. — Reconstructions  of  the  brain  of  a 7 mm.  human  embryo  (His-Prentiss).  A,  Lateral 

surface;  B,  median  sagittal  section. 


Myelencephahn 


Rhinencephalon  | Corpus  striatum  Pons 
Lamina  Icrminalis 


Spinal  cord 


Cerebral  aqueduct 
..  Mesencephalon 

isthmus 


Pallium 
Telencephalon-- 


: Cerebellum 
f Metcnccphalon 
> Rhomboid  fossa 


Cerebral  peduncle 


Diencephalon 


Fig.  253. — Brain  of  a 14  mm.  human  embryo  in  median  sagittal  section  (His  in  Sobotta). 
I,  Optic  recess:  2,  ridge  formed  by  3,  the  optic  chiasma;  4,  infundibular  recess. 


Cavities. — The  lumen  of  the  simple  neural  tube  undergoes  less  change 
than  the  walls  (Figs.  251  to  253).  The  cavity  of  the  telencephalon  extends 
into  the  paired  hemispheres  as  the  lateral  ventricles;  that  of  the  diencephalon 
(and  the  median  portion  of  the  telencephalon)  is  designated  the  third 
ventricle;  the  narrow  canal  of  the  mesencephalon  becomes  the  cerebral 


THE  BRAIN 


253 

aqueduct;  the  lumen  of  the  metencephalon  and  myelencephalon  is  the  fourth 
ventricle.  The  latter  is  continuous  with  the  central  canal  of  the  spinal 
cord. 

Flexures. — ^ While  the  several  divisions  of  the  brain  are  differentiating, 
certain  flexures  appear  in  its  roof  and  floor,  due  largely  to  unequal  growth 

a 

{Cephalic  flexure) 

Mesencephalon  Isthmus 


Fig.  254. — Brains  of  human  embryos  (after  His).  .4,  4.2  mm.  (X  20);  B,  7 mm.  (X  16): 

C,  19  mm.  (X  4). 

processes.  In  part,  these  correspond  to  those  external  bendings  seen  in 
the  head  and  neck  regions  of  young  embryos.  The  first,  or  cephalic  flexure 
appears  as  a sharp  bend  in  the  mid-brain  region  of  embryos  about  3 mm. 
long  (Figs.  61  and  251).  Soon,  the  angle  is  so  acute  that  the  long  axes  of 
the  fore-  and  hind-brains  are  nearly  parallel  (Figs.  252  and  282).  Next, 
two  other  flexures  become  evident  at  about  the  same  time.  These  are  the 


THE  CENTRAL  NERVOUS  SYSTEM 


254 


cervical  flexure  at  the  junction  of  brain  and  spinal  cord,  and  the  pontine 
flexure  in  the  region  of  the  future  pons  (Figs.  251  and  254).  The  cervical, 
like  the  cephalic  flexure,  corresponds  to  a similar  bend  in  the  gross  embryo 
(Fig.  64).  It  is  produced  by  the  entire  head  flexing  ventrad.  On  the  con- 
trary, the  ])ontine  flexure  is  peculiar  to  the  brain;  it  bends  in  the  opposite 
direction  to  the  others  and  involves  the  floor  only.  Eventually,  the 
pontine  flexure  straightens  and  disappears;  the  cervical  flexure  is  nearly 
lost,  but  the  cephalic  flexure,  somewhat  reduced,  persists. 

The  history  of  the  flexures  of  the  brain  and  the  relative  growth  of  its 
different  regions  may  be  followed  by  comparing  the  brains  of  embryos  of 
four,  five,  and  seven  weeks  (Fig.  254),  11  weeks  (Fig.  269),  and  14  weeks 
(Fig.  274).  In  the  adjoining  table  are  listed  the  primary  subdivisions  of 
the  neural  tube  and  the  parts  derived  from  them. 


DERIVATIVES  OF  THE  NEURAL  TUBE 


Primary  vesicles 

Subdivisions 

Derivatives 

Cavities 

Telencephalon 

Rhinencephalon 
Cerebral  cortex 
Corpora  striata 
Pars  optica  hypothalami 

Lateral  ventricles 
Cranial  portion  of  third 
ventricle 

Prosencephalon 

Dicncephalon 

Epithalamus 
Thalamus 
Metathalamus 
Hypothalamus 
Hypophysis 
Tuber  cinereum 
Mammillary  bodies 

Remainder  of  third  ven- 
tricle 

Mesencephalon 

Mesencephalon 

Corpora  quadrigemina 

Tegmentum 

Crura  cerebri 

Acjuaeductus  cerebri 

Rhombencephalon 

Metencephalon 

Cerebellum 

Pons 

Fourth  ventricle 

Myelencephalon 

Medulla  oblongata 

Spinal  cord 

Spinal  cord 

Central  canal. 

The  Myelencephalon. — The  wall  of  the  myelencephalon,  like  that  of 
the  spinal  cord,  differentiates  dorsally  and  ventrally  into  roof-  and  floor 
plates,  laterally  into  the  alar-  and  basal  plates  (Fig.  255).  The  boundary 
line  between  the  alar  and  basal  plates  is  the  sulcus  Umitans.  The  mye- 
lencephalon differs  from  the  spinal  cord,  however,  in  that  its  roof  plate  is 


THE  BRAIN 


255 


a broad,  thin,  and  flattened  ependymal  layer  (Figs.  255  5 and  256).  In 
the  alar  and  basal  plates,  the  marginal,  mantle,  and  ependymal  zones 
are  differentiated  as  in  the  spinal  cord  (Fig.  256).  Owing  to  the  formation 


Fig.  255. — Transverse  sections  from  a 10  mm.  human  embryo  (Prentiss).  X 44.  .4,  Through 

the  upper  spinal  cord;  B,  through  the  lower  myelencephalon. 


Fig.  256. — Transverse  sections  through  the  mj^elencephalon  of  a 10  mm.  human  embryo 
(His).  X 37.  -1,  Through  the  nuclei  of  Nn.  XI  and  A'//;  B,  through  the  nuclei  of  Nn.  A’ 

and  XII. 


of  the  pontine  flexure  at  the  beginning  of  the  second  month,  the  roof  plate 
is  broadened,  especially  in  the  cranial  portion  of  the  myelencephalon,  and 
the  alar  plates  bulge  laterally  (Figs.  257  and  258  .4).  The  cavity  of  the 
myelencephalon  is  thus  widened  from  side  to  side,  and  flattened  dorso-' 


Inner  layer 


Roof  plate 


Tractus  solitarius 


Rhombic  lip 
Resliform  body 


Spinal  tract  of 
R! . trigeminus 

Neuroblasts  from 
^ alar  plate 

Marginal  layer 


Formalio  reticularis  grisea 


Formatio  reticularis  alba 


N.  hypoglossus  Septum  medullce  N euroblasts  from  alar  plate 

(Rudiment  of  accessory  olive) 

Transverse  section  through  the  myelencephalon  of  a human  embryo  of  two  month 
(His).  X 10. 


Hemisphere 


Lobules  of  vermis 


Cerebellum 


Lateral  reces: 


Mcdidla 

'oblongata 


Rhombic  lip' 


Pyramis 


Corpora  quadrigemina 
Cerebrum  ^ 


Anlage  of  / 
vermis 

Anlage  of 
hemisphere  \ 


Rhombic  lip 


Flocculus' 


Nod  ulus 


256 


THE  CENTR.\L  NERVOUS  SYSTEM 


ventrally.  This  is  most  marked  cranially,  where,  between  the  alar 
plates  of  the  myelencephalon  and  metencephalon,  are  formed  the  lateral 


Fig.  258. — Dorsal  views  of  the  developing  cerebellum  (.4,  His;  B-D,  Prentiss).  A,  Six  weeks; 
B,  two  months;  C,  four  months;  D,  five  months. 

recesses  of  the  fourth  ventricle  (Figs.  258  A and  275).  Blood  vessels 
grow  into  the  ependymal  roof  of  the  myelencephalon,  and,  invading  the 


THE  BRAIN 


257 


lateral  recesses,  form  there  the  chorioid  plexus  of  the  fourth  ventricle. 
This  plexus  consists  of  small,  finger-like  folds  of  the  ependymal  layer  and 
its  vascular,  mesenchymal  cover.  The  line  of  attachment  of  the  epen- 
dymal layer  to  the  alar  plate  is  known  as  the  rhombic  lip  (Fig.  258  .d); 
it  becomes  later  the  tcenia  and  obex  of  the  fourth  ventricle  (B). 

In  early  stages,  the  floor  of  the  myelencephalon  is  furrowed  trans- 
versely by  the  so-called  rhombic  grooves,  six  in  number;  the  intervals 
between  successive  grooves  are  neuromeres  (cf.  Figs.  368  and  392).  Some 
view  these  as  evidential  of  a former  segmentation  of  the  head,  similar  to 
that  of  the  trunk  (p.  229).  It  is  more  probable,  however,  that  they  merely 
stand  in  relation  to  certain  cranial  nerves  and  hence  the  segmental  arrange- 
ment is  secondary. 

The  further  growth  of  the  myelencephalon  is  due : to  the  rapid  forma- 
tion of  neuroblasts,  derived  from  the  ependymal  and  mantle  layers;  to 
the  development  of  nerve  fibers  from  these  neuroblasts;  and  to  the  inva- 
sion of  fibers  from  neuroblasts  in  other  parts  of  the  brain  and  spinal  cord. 

The  neuroblasts  of  the  basal  plates  give  rise  to  the  efferent  fibers  of  the  cranial  nerves 
(Fig.  256).  In  embryos  of  the  sixth  week,  they  thus  constitute  niotor  nuclei  of  origin  for 
the  trigeminal,  abducens,  facial,  glossopharyngeal,  vagus  complex,  and  hypoglossal  nerves 
— nuclei  corresponding  to  the  ventral  and  lateral  gray  colmnns  of  the  spinal  cord.  The 
basal  plate  likewise  produces  the  reticular  formation,  which  is  derived  in  part  also  from  the 
neuroblasts  of  the  alar  plate  (Fig.  257).  Some  axons  cross  as  external  and  internal  arcuate 
fibers  and  constitute  a portion  of  the  median  longitudinal  bundle,  a fasciculus  correspond- 
ing to  the  ventral  ground  bundles  of  the  spinal  cord.  Other  axons  grow  into  the  marginal 
zone  of  the  same  side  and  form  intersegmental  fiber  tracts.  The  reticular  formation  is  thus 
differentiated  into  a gray  portion,  situated  in  the  mantle  zone,  and  into  a white  portion 
located  in  the  marginal  zone  (Fig.  257).  The  marginal  zone  is  added  to  further  by  the 
ascending  fiber  tracts  from  the  spinal  cord  and  the  descending  pyramidal  tracts  from 
the  brain.  As  in  the  cord,  the  marginal  layers  of  each  side  remain  distinct,  separated  by 
the  cells  of  the  floor  plate. 

The  alar  plates  differentiate  a httle  later  than  the  basal  plates.  The  afferent  fibers  of 
the  cranial  nerves  first  enter  the  mantle  layer,  and,  coursing  upward  and  downward,  form 
definite  tracts  (tractus  solitarius;  spinal  tract  of  fifth  nerve)  (Fig.  257).  To  these  are  added 
tracts  from  the  spinal  cord,  so  that  an  inner  gray-  and  outer  white  substance  is  formed. 
Soon,  however,  the  cells  of  the  mantle  layer  proliferate,  migrate  into  the  marginal  zone, 
and  surround  the  tracts.  These  neuroblasts  of  the  alar  plate  form  groups  of  cells  along  the 
terminal  tracts  of  the  afferent  cranial  nerves  (which  correspond  to  the  dorsal  root  fibers 
of  the  spinal  nerves)  and  constitute  the  receptive,  or  terminal  nuclei  of  the  fifth,  seventh, 
eighth,  ninth,  and  tenth  cranial  nerves.  Caudally,  the  nucleus  gracilis  and  nucleus  cunea- 
tus  are  developed  from  the  alar  plates  as  the  terminal  nuclei  for  the  afferent  fibers  which 
ascend  from  the  dorsal  funculi  of  the  spinal  cord.  The  axons  of  the  neuroblasts  in  these 
receptive  nuclei  decussate  through  the  reticular  formation,  chiefly  as  internal  arcuate  fibers, 
and  ascend  to  the  thalamus  as  the  median  lemniscus.  Still  other  nuclei  differentiate,  the 
axons  of  which  connect  the  brain  stem,  cerebellum,  and  fore-brain.  Of  these,  the  most 
conspicuous  is  the  inferior  olivary  nucleus  (Fig.  271). 


258 


THE  CENTRAL  NERVOUS  SYSTEM 


The  characteristic  form  of  the  adult  myeleneephalon  is  determined  by 
the  further  growth  of  the  above-mentioned  structures.  The  nuclei  of 
origin  of  th  cranial  nerves,  derived  from  the  basal  plate,  produce  swell- 
ings in  the  floor  of  the  fourth  ventricle  that  are  bounded  laterally  by  the 
sulcus  limitans.  The  terminal  nuclei  of  the  mixed  and  sensory  cranial 
nerves  lie  lateral  to  this  sulcus.  The  enlarged  cuneate  and  gracile  nuclei 
bound  the  ventricle  caudally  and  laterally  as  the  cuneits  and  clava  (Fig. 
275).  The  inferior  olivary  nuclei  produee  lateral,  rounded  prominences, 
the  olives,  and  ventral  to  these  are  the  large  cortico-spinal  traets,  or 
pyramids  (Fig.  271). 

The  Metencephalon. — The  alar  plates  feature  prominently  in  the 
differentiation  of  the  metencephalon.  Cranial  to  the  lateral  recesses  of 
the  fourth  ventricle,  their  cells  proliferate  ventrally  and  form  the  numerous 
and  relatively  large  nuclei  of  the  pons  (cf.  Fig.  274).  The  axons  from  the 

B 


Cerebellum 


Posterior  medullary  velum  Anterior  medullary  velum 

Fig.  259. — Median  sagittal  sections  of  the  metencephalon  (Prentiss).  A,  Two  months;  B, 

at  middle  of  fifth  month. 

cells  of  these  nuclei  mostly  cross  to  the  opposite  side  and  become  the 
hrachinm  pontis  of  the  cerebellum.  Many  cerebral  fibers  from  the  cerebral 
peduncles  end  about  the  cells  of  the  pontine  nuclei;  others  pass  through 
the  pons  as  fascicles  of  the  cortico-spinal  tracts. 

The  Cerebellum. — When  the  alar  plates  of  the  eranial  end  of  the 
myeleneephalon  are  bent  out  laterally  by  the  pontine  flexure,  their  direct 
continuations  into  the  metencephalic  region  assume  a transverse  position 
also  (Fig.  258  A).  During  the  second  month,  the  alar  plates  thicken  and 
bulge  into  the  ventricle  (Fig.  258  A).  Near  the  midline,  paired  swellings 
indicate  the  anlages  of  the  vermis,  while  the  remaining  lateral  portions 
represent  the  future  cerebellar  hemispheres  (Figs.  258  and  275). 

The  cerebellar  anlages  grow  rapidly  in  length,  so  that  their  surfaces 
are  folded  transversely.  During  the  third  month  their  walls  bulge  out- 
ward and  form  on  either  side  a convex  hemisphere  connected  with  the 


Mesencephalon 


THE  BRAIN 


259 


pons  by  the  hrachiiini  pontis  (Fig.  258  C).  In  the  meantime,  the  anlages 
of  the  vermis  have  fused  in  the  midline,  producing  a single  structure 
marked  by  transverse  fissures.  The  rhombic  lip  gives  rise  to  the  flocculus 
and  nodiiliis.  Between  the  third  and  fifth  months  the  cerebellar  cortex 
grows  faster  than  the  deeper  layers,  and  the  principal  lobes  and  fissures 
are  formed  (Fig.  258  C,  D).  The  hemispheres  are  the  last  to  be  differen- 
tiated; their  fissures  do  not  appear  until  the  fifth  month. 

The  wall  of  the  neural  tube  remains  thin  both  in  front  and  behind  the 
cerebellum;  it  constitutes  respectively  the  anterior-  and  posterior  medullary 
velum  of  the  adult  (Fig.  259  B).  The  points  of  attachment  of  the  vela 
remain  approximately  fixed,  while  the  cerebellar  cortex  grows  enormously. 
As  a result,  the  vela  are  folded  in  under  the  expanding  cerebellum. 


Fig.  260. — Transverse  sections  through  the  mesencephalon  of  a 10  mm.  human  embryo  (His). 
A,  Through  the  nucleus  of  N.  IV;  B,  through  the  nucleus  of  .V.  III. 


The  anlages  of  the  cerebellum  show  at  first  differentiation  into  the  same  three  layers 
which  are  typical  for  the  neural  tube.  During  the  second  and  third  months,  cells  from  the 
ependymal,  and  perhaps  from  the  mantle  layer  of  the  rhombic  lip  migrate  to  the  surface 
of  the  cerebellar  cortex  and  give  rise  to  the  molecular  and  granular  layers  which  are  charac- 
teristic of  the  adult  cerebellar  cortex.  The  later  dift'erentiation  of  the  cortex  is  not  com- 
pleted until  after  birth.  The  cells  of  the  granular  layer  become  unipolar  by  a process  of 
unilateral  growth.  The  axons  of  Purkinje  cells  and  those  of  entering  afferent  fibers 
form  the  deep  medullary  layer  of  the  cerebellum. 

Many  cells  of  the  mantle  layer  take  no  part  in  the  development  of  the  cerebellar 
cortex,  but  give  rise  to  neuroglia  tissue  and  to  the  internal  nuclei.  Of  these,  the  dentate 
nueleus  is  seen  at  the  end  of  the  third  month;  later,  its  cellular  layer  becomes  so  folded  as  to 
produce  characteristic  convolutions.  The  fibers  arising  from  its  cells  form  the  greater 
part  of  the  brachium  conjunctiviim. 

The  Mesencephalon. — ^Distinct  basal  and  alar  plates  can  be  recognized 
in  this  subdivision  of  the  brain,  and  each  differentiates  into  the  three 
primitive  layers  (Fig.  260).  At  the  end  of  the  first  month,  the  neuroblasts 
of  the  basal  plate  give  rise  to  the  axons  of  motor  nerves — the  oculomotor 


26o 


Till':  CENTRAL  NERVOUS  SYSTEM 


cranial  in  jiosition,  the  trochlear  caudal  (Fig.  260).  In  addition  to  these 
nuclei  of  origin,  the  red  nucleus  develops;  its  early  history  is  not  well 
understood.  The  mantle  layer  is  enclosed  ventrally  and  laterally  by 
fiber  tracts  which  develop  in  the  marginal  zone.  These  include  the  median 
and  lateral  leninisci,  and  the  descending  tracts  from  the  cerebral  cortex 
which  together  constitute  the  cerebral  peduncles. 

The  alar  plates  form  the  paired  superior  and  inferior  colliculi, 
jointly  known  as  the  corpora  quadrigemina  (Figs.  258  5 and  269).  The 
plates  thicken  and  neuroblasts  migrate  to  their  surfaces,  forming  stratified 
ganglionic  layers  comparable  to  the  cortical  layers  of  the  cerebellum  and 
the  cerebellar  nuclei.  With  the  development  of  the  superior  and  inferior 
colliculi  the  cavity  of  the  mesencephalic  region  decreases  in  size  and 
becomes  the  cerebral  aqueduct. 


Fig.  261. — Transverse  section  through  the  diencephalon  of  a 14  mm.  human  embryo  (His). 

X 29. 

The  Diencephalon. — The  wall  of  the  diencephalon  differentiates  a 
dorsal  roof  plate,  and  paired  alar  plates  which  include  both  the  lateral 
and  ventral  regions  (Fig.  261).  It  is  doubtful  if  the  basal-  and  floor  plates 
of  lower  levels  extend  into  the  diencephalon  (Kingsbury,  1922).  The 
roof  plate  becomes  a thin  ependymal  lining  to  the  folded  tela  chorioidea. 
Blood  vessels  grow  into  the  tela  and  form  the  chorioid  plexus  of  the  third 
ventricle  (Fig.  261).  At  the  junction  of  the  caudal  portion  of  the  roof 
plate  with  the  alar  |date  is  an  area  termed  the  epithalamus  (Fig.  253). 
From  it  the  epiphysis,  or  pineal  body,  evaginates  during  the  seventh  week 
(Fig.  266)  and  later  incorporates  a certain  amount  of  mesenchymal  tissue 
(Fig.  263).  The  solid,  conical  epiphysis  corresponds  but  partially  to  the 
pineal  eye  of  reptiles. 

Each  thickened  alar  plate  is  divided  by  the  sulcus  limitans  (Fig.  262) 
into  the  dorsal  thalamus  and  metathalamus  and  ventral  hypothalamus 
(Figs.  253  and  263).  The  metathalamus,  really  a part  of  the  definitive 
thalamus,  gives  rise  to  the  geniculate  bodies.  Several  structures  develop 


THE  BRAIN 


261 


from  the  hypothalamic  floor.  Passing  caudad,  these  are  the  infundibulum, 
tuber  cinereum,  and  mammillary  recess  (Fig.  262).  The  lateral  walls  of 
the  latter  enlarge  into  paired  mammillary  bodies  (Fig.  267). 


Hypothalamus 


Optic  ridge 

Fig.  262. — Median  sagittal  section  through  the  fore-  and  mid-brain  of  a 10  mm.  human 

embryo  (His). 


Diencephalon 
Chorioid  plexus 

Corpus  striatum 
Telencephalon 


Thalamus 

Pineal  body  {epithalamus) 
Cerebral  peduncle 
Cerebral  aqueduct 
' Mesencephalon 


Isthmus 
Cerebellum 
' Metcncephalon 
Rhomboid  fossa 
■ Myclcncephalon 


Lamina  terminalis  / 
Rhinencephalon 


- Spinal  cord 


Central  canal 


Fig.  263. — Median  sagittal  section  of  the  brain  from  a fetus  of  the  third  month  (His  in  Sobotta). 


The  third  ventricle  lies  largely  in  the  diencephalon  and  is  at  first  rela- 
tively broad.  Owing  to  the  thickening  of  its  lateral  walls,  it  is  compressed 
to  a narrow,  vertical  cleft  (Fig.  270).  The  thalami  are  approximated,  and 


262 


THE  CENTRAL  NERVOUS  SYSTEM 


often  fuse;  the  niassa  intermedia,  thus  formed,  is  encircled  by  the  cavity 
of  the  ventricle  (Fig.  273  B). 


Fig.  21)4. — (Jblique  section  through  the  di-  and  telencephalon  of  a 10  mm.  human  embryo 

(Prentiss).  X 61. 


csciiiephalon 


Diencephalon 


PoUimn 


Optic  stalk  Olfactory  lobe 

Fig.  265. — Lateral  view  of  the  fore-  and  mid-brain  of  a 10  mm.  human  emliryo  (His). 


The  Hypophysis. — The  hypophysis,  or  pituitary  body,  has  a double 
origin.  Its  glandular  portions  develop  from  the  ectodermal  Raihke’s 
pouch,  which  appears  at  about  3 mm.  just  in  front  of  the  pharyngeal 
membrane  (Fig.  91).  This  ])Ouch  early  comes  in  contact  with  a sac- 


THE  BRAIN 


26' 


like  extension  of  the  infimdibuhiin,  the  anlage  of  the  neural  hypophyseal 
lobe  (Figs.  262  to  264,  and  392).  Rathke’s  pouch,  at  first  flat,  grows 
laterally  and  caudally  about  the  neural  lobe,  and  loses  its  stalked  connec- 
tion with  the  oral  epithelium  at  the  end  of  the  second  month  (Fig.  415). 
The  original  cavity  of  the  pouch  becomes  the  residual  lumen  of  the  adult 
gland.  In  embryos  of  about  seven  weeks,  its  walls  differentiate  into  the 
glandular  cords  of  the  anterior  lobe.  That  portion  of  the  wall  between  the 


Longitudinal  fissure 


Fissura  prima 
Chorioid  plexus  of  lat.  ventricle 
Pallium 


Pineal  body 
Superior  colliculus 


Corpus  striatum 
Hippocampus 


Roof  plate 


Mesencephalon 


Fig.  266. — Dorsal  surface  of  the  fore-  and  mid-brain  of  a 14  nim.  human  embryo  (His).  The 
pallium  of  the  telencephalon  is  cut  away,  exposing  the  lateral  ventricle. 


lumen  and  the  neural  lobe  remains  thin  and  constitutes  the  pars  intermedia. 
Recently,  a further  glandular  portion,  the  pars  tnberalis,  has  been  recog- 
nized, lying  along  the  tuber  cinereum;  it  develops  from  the  fusion  of 
paired  lateral  lobes,  at  the  base  and  in  front  of  Rathke’s  pouch.  The 
anlage  of  the  neural  lobe  is  transformed  into  a solid  mass  of  neuroglia  tissue 
which  remains  connected  to  the  diencephalon  by  a permanent  infundibular 
stalk  (Fig.  418).  The  anterior  lobe  and  the  pars  intermedia  elaborate 
important  internal  secretions. 

The  Telencephalon. — Like  the  diencephalon,  this  specialized  division 
of  the  neural  tube  represents,  for  the  most  part,  greatly  expanded  alar 
plates.  It  is  convenient  to  regard  the  telencephalon  as  consisting  of  a me- 
dian portion,  continuous  with  the  diencephalon  and  containing  the  cranial 
part  of  the  third  ventricle,  and  of  lateral  hemispheric  outgrowths  (Fig. 


264 


THE  CENTRAL  NERVOUS  SYSTEM 


266).  Toward  the  end  of  the  first  month,  each  cerebral  hemisphere  dif- 
ferentiates into  corpus  striatum  (a  ventral  portion  continuous  with  the 
thalamus),  pallium  (primitive  cerebral  cortex),  and  rhinencephalon  (olfac- 


Maminilary  body 


Tuber  cinercum 


Pars  ant.  olf.  lobe 
Pars  post.  olj.  lobe 


Infundibulum  Optic  stalk 

Fig.  267. ■ — Lateral  view  of  the  fore-  and  mid-brain  of  a 14  mm.  human  embryo  (Ilis). 


Dicncephaloii 


M esoicc 


Pallium 


tory  brain)  (Figs.  251  and  252).  These  regions  soon  become  prominent 
(Figs.  253  and  263). 


I.ateral  ventricle 

Chorioid  ple.xus  of  lateral 
ventricle 

Thala  mils 
Corpus  striatum 

Third  ventricle 


Fig.  268. — Transverse  section  through  the  fore-brain  of  a 15  mm.  human  embryo  (His). 


The  Corpus  Striatum. — The  floor  of  each  hemisphere  produces  a 
thickening  (Fig.  252  B),  which,  at  six  weeks,  bulges  prominently  into  the 
lateral  ventricle  (Figs.  266  and  268).  The  corpus  striatum,  so  formed,  is 


THE  BRAIN 


265 


in  line  caudally  with  the  thalamus  of  the  diencephalon  and  is  closely 
connected  with  it,  both  developmentally  and  functionally.  The  corpus 
striatum  elongates  in  company  with  the  cerebral  hemisphere,  its  caudal 
portion  curving  around  to  the  tip  of  the  inferior  horn  of  the  lateral  ventricle 
and  forming  the  slender  tail  of  the  caudate  nucleus  (Figs.  269  and  272). 
The  thickening  of  the  corpus  striatum  is  due  to  an  active  proliferation  of 
cells  in  the  ependymal  layer  which  give  rise  to  a prominent  mass  of  mantle 
layer  cells.  Nerve  fibers  passing  in  both  directions  between  the  thalamus 
and  the  cerebral  cortex  course  through  the  corpus  striatum  as  laminae 


Hippocampus 

Sup.  colliculus 


Inf.  colliculus 
Cerebellum 


Pons 


Medulla  oblongata 


Caudate 

nucleus 


Internal 

capsule 


Olfactory  lobe 


Fig.  269. — The  fetal  brain  at  nearly  three  months  (His).  IMost  of  the  right  pallium  is  removed. 


which  are  arranged  in  the  form  of  a wide  V,  open  laterally.  This  V-shaped 
tract  of  white  fibers  is  the  internal  capsule.  Its  cranial  limb  partly  divides 
the  corpus  striatum  into  the  caudate  and  lenticular  nuclei;  the  caudal  limb 
of  the  capsule  extends  between  the  lenticular  nucleus  and  the  thalamus 
(Fig.  270).  The  thalamus  and  corpus  striatum  are  separated  by  a deep 
groove  until  the  end  of  the  third  month  (Fig.  268).  As  the  structures 
enlarge,  the  groove  between  them  disappears  and  they  form  one  continuous 
mass  (Fig.  270).  According  to  some  investigators  there  is  direct  fusion 
between  the  two. 


266 


THE  CENTRAL  NERVOUS  SYSTEM 


The  Pallium.  — The  i)allial  walls  expand  rapidly  until  they  overlap  and 
conceal  much  of  the  other  brain  structures  (Figs.  267,  274  and  277). 
During  this  growth  the  median  lamina  between  the  two  hemispheres  lags 
in  development,  and  thus  there  is  formed  the  longitudinal  fissure  (Fig.  266). 
The  lamina  extends  from  the  ventrally  situated  optic  chiasma  upward  and 
backward  to  the  roof  plate  of  the  diencephalon;  it  becomes  the  lamina 
tcnninalis,  the  cranial  boundary  of  the  third  ventricle  (Fig.  263).  The 
lateral  ventricles,  or  cavities  of  the  hemispheres,  at  first  communicate 


Anterior  horn 


Caudate  nucleus 

I nterventricular 
foramen 

Third  ventricle 


Lenticular  nucleus 

Column  of  fornix 
Internal  capsule 


Thalamus 


Chorioid  plexus  of 
lot.  ventricle 


Posterior  horn 


Fig.  270. — Horizontal  section  through  the  fore-brain  of  a five-months’  fetus  (His). 


broadly  with  the  third  ventricle  through  the  interventricular  foramina 
(of  Monro)  (Fig.  264).  Later,  each  foramen  is  narrowed  to  a slit,  not 
by  constriction,  but  because  its  boundaries  grow  more  slowly  than  the 
rest  of  the  telencephalon  (Fig.  268). 

The  Rliinencephalon . — During  the  sixth  week  a swelling  appears  on 
the  ventral  surface  of  each  cerebral  hemisphere  (Fig.  265).  These  enlarge 
into  distinct  olfactory  lobes,  which,  however,  remain  small  in  man  (Figs. 
267  and  271).  Each  lobe  includes  an  anterior  and  posterior  division.  The 
pars  anterior  is  the  anlage  of  the  olfactory  bulb  and  tract;  the  latter  receives 
the  backward-growing  olfactory  fibers,  and  the  original  lumen  is  lost.  The 


THE  BRAIN 


267 


pars  posterior  is  a thickening  of  the  brain  wall  which  later  constitutes  the 
anterior  perforated  substance  and  the  parolfactory  area  (Figs.  271  and  277). 

The  olfactory  apparatus  includes  also  a pallial  portion.  It  is  termed 
the  ar  chi  pallium,  because  it  forms  the  entire  primitive  wall  of  the  cerebrum, 
a condition  permanent  in  fishes  and  amphibia.  In  mammals,  the  neo- 
pallium, or  adult  cortex,  becomes  dominant  and  the  archipallium  is 
represented  by  the  hippocampus  (Figs.  266  and  269),  a portion  of  the  hippo- 
campal gyrus  (Fig.  271),  and  the  dentate  gyrus  (Fig.  272).  It  resembles 
the  rest  of  the  cerebral  cortex  in  the  arrangement  of  its  cells. 


Media)i  olfactory 
gyrus 

A liter ior  perfor- 
ated substance 
Diagonal  gyrus 


Ccrehclliim 


Insula 


Lat.  olfactory  gyrus 


Hippocampal 

gyrus 

A mygdaloid 
nucleus 


Olive 


Fig.  271. — Ventral  view  of  the  brain  of  a three-months’  fetus,  to  show  the  rhinencephalon 

(Kollmann). 


The  Chorioid  Plexus  of  the  Lateral  Ventricles. — ^Just  as  the  chorioid 
plexus  of  the  third  ventricle  develops  in  the  folds  of  the  roof  plate  of  the 
dieneephalon,  so  the  thin,  median  wall  of  the  pallium,  at  its  junction  with 
the  wall  of  the  diencephalon,  is  folded  into  each  lateral  ventricle.  A 
vascular  plexus,  continuous  with  that  of  the  third  ventricle,  grows  into 
this  fold,  and  projects  into  the  corresponding  lateral  ventricle  (Figs.  266 
and  268).  The  fold  of  the  pallial  wall  forms  the  chorioid  fissure  (Fig.  269), 
and  the  vascular  plexus  is  the  chorioid  plexus  of  the  lateral  ventricle 
(Fig.  272).  This  is  a paired  structure,  which,  with  the  plexus  of  the  third 
ventricle,  makes  a T-shaped  figure,  the  stem  of  the  T overlying  the  third 
ventricle  and  its  curved  arms  projecting  into  the  lateral  ventricles  just 
caudal  to  the  interventricular  foramen.  Later,  as  the  pallium  expands, 


268 


THE  CENTRAL  NERVOUS  SYSTEM 


the  chorioid  plexus  of  the  lateral  ventricles  and  the  chorioidal  fissures  are 
elongated  extensively  into  the  temporal  lobes  and  inferior  horns  of  the 
lateral  ventricles  (Figs.  270  and  272). 

Commissures  of  the  Telencephalon. — The  important  commissures  are 
the  fornix,  anterior  commissure,  and  corpus  callosum.  The  first  two  are 
older  commissures  of  the  archipallium,  while  the  larger  corpus  callosum 
is  the  great  transverse  bridge  of  the  neopallium,  or  cerebral  cortex.  The 
commissures  develop  in  relation  to  the  lamina  terminalis,  crossing  partly 
in  its  wall  and  partly  in  the  fused  adjacent  portions  of  the  median  pallial 
walls.  Owing  to  the  union  of  the  pallial  walls  dorsal  and  cranial  to  it, 
the  lamina  thickens  rapidly  during  the  fourth  and  fifth  months.  It  is  at 
this  time  that  the  significant  development  of  the  commissures  occurs. 


Falx 


Fi<;.  272. — Transverse  section  through  the  telencephalon  of  a three-months’  fetus  (His). 

Th,  Thalamus;  Cs,  corpus  striatum. 

The  fornix  takes  its  origin  early,  chiefly  from  cells  in  the  hippocampus. 
The  fibers  course  along  the  chorioidal  side  of  the  hippocampus  cranially 
(cf.  Fig.  269),  passing  dorsal  to  the  foramen  of  Monro  (Fig.  273  A). 
In  the  cranial  portion  of  the  lamina  terminalis,  fibers  are  both  given  off  to 
the  basal  portion  of  the  rhinencephalon  and  received  from  it.  In  this  re- 
gion, fibers  crossing  the  midplane  form  the  hippocampal  commissure  (Fig. 
273  A)\  with  the  later  growth  of  the  corpus  callosum  it  shifts  further  caudad 
(Fig.  272  B).  Other  fibers,  as  the  diverging  columns  of  the  fornix,  curve 
ventrad  and  end  in  the  mammillary  body  of  the  hypothalamus  (Fig.  273  B). 

The  fibers  of  the  anterior  commissure  cross  in  the  lamina  terminalis, 
ventral  to  the  primitive  hippocampal  commissure  (Fig.  273  ^4).  They  arise 
in  paired  cranial  and  caudal  divisions.  The  fibers  of  the  former  intercon- 


THE  BRAIN 


269 


nect  the  olfactory  bulbs  in  a horse-shoe  bow.  The  fibers  of  the  caudal  divi- 
sion pass  ventrally  between  the  corpora  striata  and  the  cortex,  and  may 
be  derived  from  one  or  both  of  these  regions. 

The  corpus  callosum  appears,  cranial  and  dorsal  to  the  primitive 
hippocampal  commissure,  in  the  roof  of  the  thickened  lamina  terminalis 
(Fig.  273  A).  Through  its  fibers,  which  arise  from  neuroblasts  in  the  wall 


Corpus  callosum 


Body  of  fornix 


Chorioid  fissure 
Thala  miis 


Column  of  fornix 


Hippocampal  commissure 

Anterior  commissure 


B 

Body  of  fornix  Hippocampal  commissure 
Septum  pcllucidum 


Corpus  callosum 


Ant.  commissure 


Column  of  fornix 


Thalamus 
(Massa  intermedia) 


Fig.  273. — The  cerebral  commissures  in  median  section  (adapted  by  Prentiss),  .1,  Three 

months;  B,  four  months. 


of  the  neopallium,  nearly  all  regions  of  one  hemisphere  are  associated 
eventually  with  corresponding  regions  of  the  other.  The  fibers,  found 
first  in  the  corpus  callosum,  arise  in  the  median  wall  of  the  hemispheres. 
AsHhe  pallium  expands,  interstitial  fibers  develop  which  extend  the  corpus 
callosum  both  cranially  and  caudally  (Fig.  273  B).  In  fetuses  of  five 
months,  this  great  commissure  is  a conspicuous  structure  and  shows  the 
form  which  is  characteristic  of  the  adult  (Figs.  272  5 and  277). 


THE  CENTRAL  NERVOUS  SYSTEM 


270 


The  triangular  interval  between  the  fornix  and  corpus  callosum 
contains  a thin  partition  which  separates  the  two  lateral  ventricles  (Fig. 
273  B).  This  scplitm  pelluciditm  is  a membranous  portion  of  the  lamina 
terminalis  and  really  is  thinned,  median  pallial  wall.  As  a result  of  stretch- 
ing, caused  by  the  growth  of  the  corpus  callosum,  a cavity  sometimes 
forms  between  the  laminae  of  the  septum;  it  is  designated  the  space  of 
the  septum  pcUncidiim,  or  often,  falsely,  the^///z  ventricle  (Fig.  277). 


Parietal  lohe 


Lateral 

fissure 


Frontal 

lobe 


Temporal 

lobe 

Pons 


Occipital 

lobe 


Cerebellum 


Myelenceph- 

alon 


Spinal  cord 


\ 


II 


Fig.  374. ^Lateral  view  of  the  l>rain  in  situ,  at  the  middle  of  the  fourth  month  (His.) 

External  Configuration  of  the  Hemispheres. — The  telencephalon  so  ’ 
expands  cranially,  caudally,  and  ventrally  that  four  lobes  may  be  dis- 
tinguished (Fig.  274):  (i)  a cranial  frontal  lobe;  (2)  a dorsal  parietal  lobe; 
(3)  a caudal  occipital  lobe;  and  (4)  a ventro-lateral  temporal  lobe.  The 
ventricle  extends  into  each  of  these  regions  and  forms  respectively 
the  anterior  horn,  the  body,  the  posterior  horn,  and  the  inferior  horn  of  the 
lateral  ventricle. 

The  surface  extent  of  the  cerebral  wall,  the  thin  gray  cortex,  increases 
more  rapidly  than  the  underlying,  white  medullary  layer.  As  a result. 


a 


THE  BRAIN 


271 


the  cortex  is  folded,  producing  convolutions,  between  which  are  prominent 
furrows,  termed  fissures.  The  chorioid  fissure  is  formed,  as  already  ex- 


Parietal  lobe 


Corpora 

qiiadrigemina 


Hemisphere  of 
cerebellum 


Vermis  of  cerebelliar 


Impression  of 
thalamus 

Temporal  lobe 


Lateral  recess  of 
fourth  ventricle 

Fasciculus  gracilis 
Medulla  oblongata 


Fig.  275. — Dorsal  view  of  the  brain  from  a three  months’  fetus  (Kollmann). 


Postcentral  sulcus  Central  sulcus 


Parietal 

lobe 

Supra- 
marifinal  \ 
and  an-  "j 
gnlar  fyri  ' 

Lateral 

fissure 


Middle 
lent  poral 
sulcus 


Occipital 

lobe 


Inferior 

frontal 

sulcus 


I Lateral 
I fissure 


Temporal 

lobe 


Superior  temporal  gyrus  Middle  temporal  gyrus 


Fig.  276. — Lateral  view  of  the  right  cerebral  hemisphere  from  a seven-months’  fetus 

(Kollmann). 


plained  (p.  267),  by  the  ingrowth  of  the  chorioid  plexus  (Fig.  267).  During 
the  third  month,  the  rhinal  (Fig.  277)  and  hippocampal  fissures  develop  in 


272 


THE  CENTRAL  NERVOUS  SYSTEM 


association  with  the  rhinencephalon.  The  latter  fissure  represents  a 
curved  infolding  along  the  median  wall  of  the  temporal  lobe  (Fig.  272); 
the  corresponding  elevation  on  the  inner  surface  of  the  pallium  is  the 
hippocampus  (Figs.  266  and  269).  At  the  same  time,  the  lateral  fissure 
(of  Sylvius)  makes  its  appearance  in  the  following  way  (Fig.  274):  The 

cortex  overlying  the  corpus  striatum  develops  more  slowly  than  the  sur- 
rounding areas  and  is  thus  gradually  overgrown  by  opercular  folds  of 
the  frontal,  parietal,  and  temporal  lobes.  The  area  thus  covered  is  the 
insula  (island  of  Reil),  and  the  depression  so  formed  is  the  lateral  fissure 
(Fig.  276).  These  opercula  are  not  approximated  over  the  insula  until 
after  birth. 


Corpus  callosum 
Gyrus  cinguli 


Sulcus  of  corpus  callosum 
Splenium 

Paricto-occipital  fissure 


Cal- 

carine 

fissure 


Cune.us 


Olfactory  lobe 

Optic  nerve 


KJnnal  fissun 
Temporal  lobe 


Fig.  277. — Median  surface  of  the  right  cerebral  hemisphere  from  a seven-months’  fetus 

(Kollmann). 


Space  of 
septum 
pellu- 
cidum 
Lamina 
ierniinalis 


Parol- 

factory 

area 


In  fetuses  of  six  to  seven  months,  four  other  neopallial  depressions  ; 
appear  which  later  form  important  landmarks  in  the  cerebral  topography.  ( 
They  are:  (i)  the  central  sulcus,  or  fissure  of  Rolando,  which  forms  the  ^ 
dorso-lateral  boundary  line  between  the  frontal  and  parietal  lobes  (Fig.  '* 
276);  (2)  the  parieto-occipital  fissure,  which,  on  the  median  wall  of  the 
cerebrum,  is  the  line  of  separation  between  the  occipital  and  parietal 
lobes  (Fig.  277);  (3)  the  calcarine  fissure,  which  marks  the  position  of  the  • 
visual  area  of  the  cerebrum  (Fig.  277)  and  internally  causes  the  convexity 
termed  the  calcar  avis;  (4)  the  collateral  fissure  on  the  ventral  surface  of  ■ 
the  tem])oral  lobe,  which  produces  the  inward  bulging  on  the  floor  of  the 
posterior  horn  of  the  ventricle  known  as  the  collateral  eminence. 


THE  BRAIN 


273 


Simultaneously  with  the  development  of  the  latter  group  of  fissures/ 
appear  other  shallower  depressions  known  as  sulci.  These  have  a definite 
arrangement,  and,  with  the  fissures,  mark  off  from  each  other  the  various 
functional  areas  of  the  cerebrum.  The  surface  convolutions  between  the 
depressions  constitute  the  gyri  of  the  adult  cerebrum. 

Histogenesis  of  the  Cerebral  Cortex. — In  the  wall  of  the  pallium  are 
differentiated  the  three  primitive  zones  typical  of  the  neural  tube;  the 
ependymal,  mantle,  and  marginal  layers.  During  the  first  two  months 
the  cortex  remains  thin  and  differentiation  is  slow.  At  eight  weeks, 
neuroblasts  migrate  from  the  ependymal  and  mantle  zones  into  the  super- 
ficial marginal  zone  and  give  rise  to  layers  of  pyramidal  and  other  cells 
typical  of  the  cerebrum.  The  differentiation  of  these  layers  is  most  active 
during  the  third  and  fourth  months,  but  probably  continues  until  after 
birth.  From  the  fourth  month  on,  the  cerebral  wall  thickens  rapidly, 
owing  to  the  development  of  fibers  from  the  thalamus  and  corpus  striatum, 
and  of  endogenous  fibers  from  the  neuroblasts  of  the  cortex.  The  fibers 
form  a white,  inner  medullary  layer,  surrounded  by  the  gray  cortex. 
Myelination  begins  shortly  before  birth,  but  some  fibers  may  not  acquire 
their  sheaths  until  after  the  twentieth  year.  As  the  cerebral  wall  increases 
in  thickness,  the  size  of  the  lateral  ventricle  relatively  diminishes;  espe- 
cially is  this  true  of  its  lateral  diameter. 

Anomalies. — There  are  numerous  types  of  defective  neural  tube  development — most 
the  result  of  arrest.  These  usually  involve  the  bony  investments  as  well,  and  produce 
conspicuous  malformations. 

The  more  or  less  extensive  failure  of  the  neural  groove  to  close  produces  cranioschisis 
or  rachischisis,  depending  on  whether  the  region  of  the  head  or  vertebral  column  is  affected. 
In  such  instances  the  roof  of  the  skull  is  lacking  [acrania;  liemicrania),  or  there  are  clefts 
in  the  vertebral  canal.  If  the  defect  contains  a sac-like  protrusion  of  the  membranes,  the 
condition  is  known  as  meningocoelc;  if  the  neural  wall  alone  protrudes,  it  is  cncephaloccele 
(brain)  or  myeloccele  (spinal  cord) ; if,  as  is  most  common,  both  are  involved,  it  is  meningo- 
encepJialoccele,  or  meningo-myeloccele.  Such  a hernial  condition  of  the  spine  is  often  called 
spina  bifida  and  is  most  frequent  in  the  lumbo-sacral  region,  where  the  sac  may  become  the 
size  of  a child’s  head. 

An  excessive  fluid  content  in  the  brain  cavities  causes  both  brain  and  skull  to  enlarge, 
producing  hydrocephaly  The  virtual  absence  of  a brain  is  anencephaly;  of  the  spinal  cord, 
a my  el  us. 

18 


CHAPTER  XIV 

THE  PERIPHERAL  NERVOUS  SYSTEM 

I'he  peripheral  nervous  system  consists  of  bundles  of  myelinated  and 
unmyelinated  nerve  fibers,  and  aggregations  of  nerve  , cells,  the  ganglia. 
The  fibers  are  of  two  types:  afferent  fibers,  which  carry  sensory  impulses 
to  the  central  nervous  system,  and  efferent  fibers,  which  carry  motor 


Special  somatic  a fferent 
nucleus 

General  somatic  afferent, 
n itcleus 
Alar  plate' 

Visceral  afferent  nucleus 

General  visceral  efferent 
nucleus 

Special  visceral  efferent — 
nucleus 
Basal  plate 

Somatic  efferent  nucleus^ 


Somatic  muscle 
Sympathetic  ganglion- 
Visceral  mucous  membrane . 

Smooth  muscle 


''  Sensory  ganglion 
Branchial  muscle 


Fig.  278. — Diagrammatic  section  through  the  embryonic  myelencephalon,  showing  the 
arrangement  of  the  functional  cell  columns  and  the  origin,  course  and  termination  of  the  func- 
tional components  of  the  cranial  nerves  (Ranson). 

impulses  away  from  the  nervous  centers.  The  peripheral  afferent  fibers 
originate  from  nerve  cells  located  in  the  ganglion  crest  (p.  241)  outside 
the  neural  tube.  The  peripheral  efferent  fibers  grow  from  neuroblasts  of 
the  basal  plate  and  emerge  ventro-laterally  out  of  the  neural  tube.  Fibers 
of  one  or  both  sorts  converge  into  distinct  segmental  cords  called  nerves. 
These  belong  to  two  main  systems:  the  cerebro-spinal  series  and  the 
sympathetic  division. 


274 


THE  SPINAL  NERVES 


275 


Functional  Classification  of  Fibers. — -The  early  observation  that  sen- 
sory impulses  travel  in  the  dorsal  root  fibers  and  motor  impulses  in  ventral 
root  fibers  (Fig.  281)  has  been  supplemented  by  a more  complete  analysis 
(Fig.  278).  All  neurons  fall  within  four  chief  functional  groups,  which 
are  in  turn  subdivided  as  indicated  in  the  accompanying  table.  No  single 
nerve  contains  representatives  of  every  fiber  type;  those  components 
designated  ‘special’  are  peculiar  to  the  cranial  nerves  alone. 

1.  Somatic  afferent. 

(а)  General  (Fibers  ending  chiefly  in  the  integument). 

(б)  Special  (Neurons  of  the  eye  and  ear). 

2.  Visceral  afferent. 

(а)  General  (Sympathetic  fibers  conducting  sensory  impulses  from 
the  viscera). 

(б)  Special  (Fibers  to  the  olfactory  membrane  and  taste  buds). 

3.  Somatic  efferent.  (Fibers  ending  on  skeletal  muscle). 

4.  Visceral  efferent. 

(а)  General  (Fibers  ending  about  sympathetic  ganglion  cells,  which 
in  turn  control  smooth  and  cardiac  muscle  and  glandular  tissue). 

(б)  Special  (Cranial  nerve  fibers  ending  directly  on  striated, 
branchial-arch  musculature) . 

THE  SPINAL  NERVES 

The  spinal  nerves  are  arranged  segmentally,  and  each  is  attached  to 
the  spinal  cord  by  a dorsal  (posterior)  root,  with  which  is  associated  a 

N.  IX  N.  X-XI  gangtion  crest 


Fig.  279. — The  cerebro-spinal  nerves  of  a 4 mm.  human  embryo  (Streeter).  X 17.  Ci-6, 

Ventral  roots  of  cervical  spinal  nerves. 

spinal  ganglion,  and  a ventral  (anterior)  root  (Fig.  246).  In  embryos  of 
4 mm.,  the  ventral  roots  are  already  developing  as  outgrowths  of  neuro- 
blasts in  the  mantle  layer  of  the  spinal  cord  (Fig.  279).  The  spinal 
ganglia  are  represented  as  enlargements  along  the  continuous  ganglion 
crest.  At  the  stage  of  7 mm.,  or  five  weeks,  the  cells  of  the  spinal  ganglia 
begin  to  develop  centrally  directed  processes  which  enter  the  marginal 


276 


THE  PERIPHERAL  NERVOUS  SYSTEM 


Dorsal  root 


Fig.  2S0. — The  cerebro-spinal  nerves  of  a 7 ihiti.  human  embryo  (Streeter).  X i7- 

Marginal  layer 

Ependymal  layer 
Mantle  layer 


Somatic  sensory  neuron 
Visceral  sensory  nemo 


Visceral  motor 
Somatic  motor 
Dorsal 


Spinal  cord 


Lat. 


division 


Ventral  terminal  division  op  J V \ Aorta 

spinal  nerve  / \ 

Ramus  communicans  Sympathetic  ganglion 

Pj,;  281. — Transverse  section  of  a 10  mm.  human  embryo,  showing  a spinal  nerve  and 

functional  components  (Prentiss). 


N.  IX-XI  ganglion  crest 


N.  ophthahnicus 

N.  inaxilluris 
N.  niasticatorius 

N.  imuid ihularis 


Dorsal 
root  fibers 


THE  SPINAL  NERVES 


277 


zone  of  the  cord  as  dorsal  root  fibers  (Fig.  280).  Peripheral  processes  of 
the  ganglion  cells  soon  join  the  ventral  root  fibers  in  the  trunk  of  the 
nerve  (Fig.  246). 

At  10  mm.  (Fig.  282),  the  cellular  bridges  of  the  ganglion  crest,  which 
hitherto  interconnect  spinal  ganglia,  have  begun  to  disappear,  and  the 


Gang,  acusticum 
Gang,  semilunarc  n.  V 

Cerebellum  I N.Vl; 


esicula  auditiva 
Gang,  radicis  n.  IX 
■Gang,  petrosiim 
; Gang,  radicis  n.X 


Gang.  Froriep 


Tubus  digest. 


R.  posterior 

R.  terminal  is  laleralis 

IS.  ! ( ' : R.  tcrminalis  anterior 

N . femoral  j i \ I L. 

N'.  obturator  ' Mesonephros 

Xn.  ilioing.  et  hypogastr. 


N.  TII 
N.  IV 


N.  hypo  gloss  us 


X . frontalis 


-----  N.  XI 


Gang,  nodos. 


_N.  desc.  cerv. 
..Rami  hyoid. 

(Ansa  hypoglossi) 
N.  M usculoiuian. 
" N.  axillaris 
"N.  pkrenicus 

--N.  medianus 
■-N.  radialis 
~'N.  ulnaris 


Diaphragma  ' 
He  par 
I Co. 


N.  tibialis  '' 


/V.  peroneus  ' 


N,  nasociliaris  ■" 


N.  maxillaris  ' 

N . niandibularis 
Gang,  genicidatum 
N.  chorda  tympani 
Cor 


Fig.  282. — The  nervous  system  of  a 10  mm.  human  embryo  (Streeter).  X 12. 


several  parts  of  a typical  spinal  nerve  are  evident  (Figs.  246  and  281). 
The  trunk  of  the  nerve,  just  ventral  to  the  union  of  the  dorsal  and  ventral 
roots,  gives  off  laterally  the  dorsal  ramus,  the  fibers  of  which  supply  the 
dorsal  muscles  and  integument.  The  ventral  ramus,  continuing,  gives 
off  mesially  the  ramus  communicans  to  the  sympathetic  ganglion,  and 
divides  into  the  lateral  and  ventral  terminal  rami.  The  efferent  fibers  of 


278 


THE  PERIPHERAL  NERVOUS  SYSTEM 


these  rami  supply  the  muscles  of  the  lateral  and  ventral  body  wall,  and 
the  afferent  fibers  end  in  the  integument  of  the  same  regions. 

Nerve  Plexuses. — ^At  the  points  where  the  anterior  and  lateral  ter- 
minal rami  arise,  connecting  loops  may  extend  from  one  spinal  nerve  to 
another.  Thus,  in  the  neck  region,  superficial  and  deep  cervical  plexuses 
are  formed.  The  deep  cervical  jilexus  gives  rise  to  the  ansa  hypoglossi 
and  the  phrenic  nerve  (Fig.  282). 

d'he  nerves  supplying  the  arm  and  leg  also  form  plexuses  that  first 
appear  at  7 mm.  (Fig.  280)  and  are  clearly  indicated  in  embryos  of  10 
mm.  (Fig.  282).  The  trunks  of  the  last  four  cervical  nerves  and  of  the 
first  thoracic  unite  into  a flattened  ])late,  the  anlage  of  the  brachial  plexus. 
From  this  ]>late  nervous  cords  extend  into  the  intermuscular  spaces  and 
end  in  the  premuscle  masses.  The  developing  skeleton  of  the  shoulder 
splits  the  lirachial  plexus  into  dorsal  and  ventral  laminae.  From  the 
dorsal  lamina  arise  the  musculo-cutaneous,  median,  and  ulna  nerves; 
from  the  ventral  lamina,  the  axillary  and  radial  nerves. 

The  lumbar  and  sacral  nerves  to  the  leg  unite  in  a plate-like  structure, 
the  anlage  of  the  lumbosacral  plexus  (Fig.  282).  The  plate  is  divided  by 
the  skeletal  elements  of  the  pelvis  and  femur  into  two  lateral  and  two 
median  trunks.  Of  the  cranial  pair,  the  lateral  becomes  the  femoral 
nerve;  the  median,  the  obturator  nerve.  The  caudal  pair  constitutes 
the  ])rimitive  sciatic  nerve;  the  lateral  trunk  will  be  the  peroneal  nerve, 
the  median  trunk  the  tibial. 


Twelve  pairs  of  cranial  nerves  appear  at  about  the  end  of  the  first 
month.  They  are  not  arranged  segmentally  and  attempts  to  interpret 
them  as  serial  homologues  of  spinal  nerves  fail.  In  addition  to  the  general 
sensory  and  motor  components  of  spinal  nerves,  the  cranial  group  contains 
special  fibers  to  the  major  sense  organs  and  to  muscles  derived  from 
branchial  arches.  The  several  sensory  and  motor  nuclei  are  arranged 
in  definite  longitudinal  columns  within  their  respective  alar  and  basal 
plates  (Fig.  278).  Unlike  the  spinal  series,  the  cranial  nerves  vary  widely 
in  functional  composition.  Those  of  the  first  two  groups  in  the  subjoined 
table  have  but  a single  kind  of  fiber;  on  the  contrary,  the  members  of  the 
third  group  are  all  mixed,  as  witness  the  ninth  and  tenth  which  contain 
five  different  types  each.  The  cranial  nerves  fall  roughly  into  three  func- 
tional groups; 


THE  CRANIAL  NERVES 


Special 

Sensouv 


Somatic 

Motor 


Visceral  Sensory 


AND  Motor 


I.  Olfactory. 
II.  Optic. 


III.  Oculomotor. 


V.  Trigeminal. 

VII.  Facial. 

IX.  Glossopharyngeal. 

X.  Vagus  comple.x  (including 
XI.  Spinal  Accessory). 


VIII.  Acoustic. 


IV.  Trochlear. 
VI.  Abducens. 


XII.  Hypoglossal. 


THE  CRANIAL  NERVES 


279 


(A)  The  Special  Sensory  Nerves 

1.  The  Olfactory  Nerve,  though  purely  sensory,  has  no  ganglion. 
Its  nerve  cells  lie  at  first  in  the  epithelium  of  the  nose  and  are  of  the 
bipolar  type.  From  them,  peripheral  processes  develop  which  end  directly 
at  the  surface  of  the  olfactory  epithelium  (Fig.  283).  Central  processes 
grow  backward  during  the  fifth  week  and  form  the  strands  of  the  olfactory 
nerve,  around  which  the  cribriform  plate  later  develops.  They  end  in 
the  glomeruli  of  the  olfactory  bulb  in  contact  with  dendrites  of  the 
mitral  cells,  or  olfactory  neurons  of  the  second  order.  Some  olfactory  cells 
migrate  from  the  epithelium,  with  which,  however,  they  retain  periph- 
eral connections.  Such  bipolar  elements,  found  along  the  entire  course 
of  the  nerve,  resemble  ordinary  dorsal  ganglion  cells.  The  olfactory 
nerve  fibers  are  peculiar  in  that  they  remain  unmyelinated.  Nerve 
fibers  from  the  epithelium  of  the  vestigial  vomero-nasal  organ  (of  Jacobson) 
also  end  in  the  olfactory  bulb. 

The  ganglionated  terminal  nerve 
courses  in  close  association  with  the 
olfactory  nerve.  Its  unmyelinated 
fibers  end  in  the  epithelium  of  the 
vomero-nasal  organ  and  of  the  septum. 

Although  evidently  a distinct  nerve,  its 
relations  and  significance  are  obscure. 

2.  The  Optic  Nerve  is  formed  by 
fibers  which  grow  from  neuroblasts  in 
the  nervous  layer  of  the  retina.  Since 
the  retina  differentiates  from  the 
evaginated  wall  of  the  fore-brain  (Fig.  264),  the  optic  nerve  is  not  a 
true  peripheral  nerve,  but  belongs  to  the  central  system  of  tracts.  The 
neuroblasts  from  which  the  optic  nerve  fibers  develop  constitute  the 
ganglion  cell  layer  of  the  retina  (Fig.  301).  During  the  sixth  and  seventh 
weeks  these  cells  give  rise  to  central  processes  which  form  a nerve  fiber 
layer  on  the  inner  side  of  the  retina.  The  optic  fibers  converge  to  the  optic 
stalk  and  grow  through  its  wall  back  to  the  brain  (Fig.  284  A).  The 
cells  of  the  optic  stalk  are  converted  into  a neuroglia  framework  and  its 
cavity  is  obliterated  {B).  In  the  floor  of  the  fore-brain,  at  the  boundar}" 
between  telencephalon  and  diencephalon,  the  fibers  from  the  median  half 
of  each  retina  at  about  the  end  of  the  second  month  cross  to  the  opposite 
side,  and  this  decussation  constitutes  the  optic  cliiasma.  The  crossed 
and  uncrossed  fibers  constitute  the  optic  tract  (Fig.  271). 

Efferent  fibers,  terminating  in  the  inner  reticular  layer  of  the  retina, 
are  present  also.  In  certain  fishes,  where  their  function  has  been  studied, 
these  fibers  resemble  visceral  efferent  components  (Arey,  1916). 


Olfactory  tract 
-Mitral  cell 

Glomerulus 


pCribriform  plate 
- Olfactory  nerve  fiber 

"^'Olfactory  epithelium 

Fig.  283. — Diagram  of  the  relations  of 
the  fibers  in  the  olfactory  nerve. 


28o 


THE  PERIPHERAL  NERVOUS  SYSTEM 


8.  The  Acoustic  Nerve  is  composed  of  fibers  which  grow  from  the 
acoustic  ganglion.  Its  cells  arise  directly  from  the  brain  wall  of  2 mm. 
embryos  (Bartelmez,  1922)  and  soon  lie  just  cranial  to  the  otic  vesicle 
(Fig.  305).  The  cells  become  bipolar,  central  processes  uniting  the 
ganglion  to  the  tuberculum  acusticum  of  the  myelencephalon  and 
peripheral  fibers  connecting  it  with  the  wall  of  the  otocyst. 

The  acoustic  ganglion  is  differentiated  into  vestibula?-  and  spiral 
ganglia  (Fig.  285).  The  original  ganglion  elongates  and  is  subdivided 


Fig.  284. — Transverse  sections  through  the  human  optic  stalk  during  its  transformation  into 
the  optic  nerve  (redrawn  after  Bach  and  Seefelder).  /I,  14.5  mm.;  B,  19  mm. 


into  superior  and  inferior  portions  in  7 mm.  embryos.  The  superior  part 
supplies  fibers  to  the  utriculus  and  to  the  ampullae  of  the  anterior  and 
lateral  semicircular  ducts.  Part  of  the  inferior  portion  innervates  the 
sacculus  and  the  ampulla  of  the  posterior  semicircular  duct,  and  this 
portion,  together  with  the  entire  pars  superior,  constitutes  the  vestibular 
ganglion.  Most  of  the  pars  inferior,  however,  differentiates  into  the 
spiral  ganglion,  the  peripheral  fibers  of  which  innervate  the  hair  cells  of 
the  spiral  organ  (of  Corti)  in  the  cochlea.  The  spiral  ganglion  appears  in 
9 mm.  embryos  and  conforms  to  the  spiral  turns  of  the  cochlea,  hence  its 
name.  Its  central  nerve  fibers  form  the  cochlear  division  of  the  acoustic 
nerve.  This  is  distinctly  separated  from  the  central  fibers  of  the  vesti- 
bular ganglion  which  constitute  the  vestibular  division  of  the  acoustic 
nerve,  the  fibers  of  which  are  equilibratory  in  function.  The  pars 
inferior  of  the  vestibular  ganglion  becomes  closely  connected  with  the  n. 
cochlearis,  and  thus  in  the  adult  it  appears  as  though  the  sacculus  and 
posterior  ampulla  were  supplied  by  the  cochlear  nerve. 


THE  CRANIAL  NERVES 


281 


,'N.  vestib. 

JX  t — Pars.  sup. 

I '}  4 mm.  y mm 

\^^--Pars.  inf. 


Pars.  sup.  Pars.  sup. 

Pars.  inf.  '\  . • N.  vestib.  - ' .Pars.  inf. 

WW  /! 


. • . 'N.  cock 


g mm. 


Gang,  spirale 


R.  amp.  sup.  R-  ™P-  ^’<P- 


I'lG.  285. — The  development  of  the  acoustic  ganglion  and  nerve  (Streeter).  The  vestibular 
ganglion  is  finely  stippled,  the  spiral  ganglion  coarsely  stippled. 


282 


THE  PERIPHERAL  NERVOUS  SYSTEM 


(B)  The  Somatic  Motor  Nerves 

This  group,  consisting  of  the  three  nerves  to  the  eye  muscles  and  the 
n.  hypoglossus,  is  purely  motor,  the  fibers  originating  from  neuroblasts 
of  the  basal  plate  of  the  brain  stem,  near  the  midplane.  They  are  regarded 
as  homologues  of  the  ventral  motor  roots  of  the  spinal  cord,  but  they  have 
lost  their  segmental  arrangement  and  are  otherwise  modified.  The 
nuclei  of  origin  of  these  nerves  are  colored  red  in  Fig.  287. 

3.  The  Oculomotor  Nerve  develops  from  neuroblasts  in  the  basal 
plate  of  the  mesencephalon  (Fig.  260  B).  The  fibers  emerge  as  small 
fascicles  on  the  ventral  surface  of  the  mid-brain,  in  the  concavity  due  to 
the  cephalic  flexure  (Figs.  282  and  287).  The  fascicles  converge,  form  the 
trunk  of  the  nerve,  and  end  in  the  premuscle  masses  of  the  eye.  The 
nerve  eventually  supplies  all  of  the  extrinsic  muscles  of  the  eye,  save 
the  superior  oblique  and  external  rectus. 

4.  The  Trochlear  Nerve  fibers  arise  from  neuroblasts  of  the  basal 
plate,  located  just  caudal  to  the  nucleus  of  origin  of  the  oculomotor 
nerve  (Fig.  287).  They  are  directed  dorsad,  curve  around  the  cerebral 
aqueduct,  and,  crossing  in  its  roof,  emerge  at  the  isthmus  (Fig.  260  A). 
From  this  superficial  origin,  each  passes  ventrad  as  a slender  nerve  which 
connects  with  the  anlage  of  the  superior  oblique  muscle  of  the  eye 
(Fig.  282). 

6.  The  Abducens  Nerve  takes  origin  from  a nucleus  of  cells  in  the  basal 
plate  of  the  myelencephalon,  located  directly  beneath  the  fourth  neuro- 
mere  (Figs.  282  and  287).  The  converging  fibers  emerge  ventrally  at  a 
point  caudal  to  the  future  pons,  and,  as  a single  trunk,  course  cranially, 
mesial  to  the  semilunar  ganglion,  finally  ending  in  the  anlage  of  the 
external  rectus  muscle  of  the  eye.  Vestigial  rootlets  of  the  abducens  and 
hypoglossal  nerve  tend  to  fill  in  the  gap  between  these  two  nerves. 

12.  The  Hypoglossal  Nerve  results  from  the  fusion  of  the  ventral  root 
fibers  of  three  to  five  precervical  nerves.  Its  fibers  originate  from  neuro- 
blasts of  the  basal  plate  and  emerge  from  the  ventral  wall  of  the  myelen- 
cephalon in  several  groups  (Figs.  279  and  287).  In  embryos  of  7 mm., 
the  fibers  have  converged  ventrally  to  form  the  trunk  of  the  nerve 
(Fig.  289).  Later,  they  grow  cranially,  lateral  to  the  ganglion  nodosum, 
and  eventually  end  in  the  muscle  fibers  of  the  tongue  (Fig.  282).^  The 
nerve  in  its  development  unites  with  the  first  three  cervical  nerves  to  form 
the  ansa  hypoglossi. 

That  the  hypoglossus  is  a composite  nerve,  homologous  with  the  ventral  roots  of  the 
spinal  nerves,  is  shown:  (i)  by  the  segmental  origin  of  its  fibers;  (2)  from  the  fact  that  its 
nucleus  of  origin  is  a cranial  continuation  of  the  ventral  gray  column,  or  nucleus  of  origin 
for  the  ventral  spinal  roots;  (3)  from  the  fact  that  in  mammalian  embryos  (pig;  sheep;  cat; 
etc.)  rudimentary  dorsal  ganglia  are  developed,  one  of  which  at  least  (Froriep’s  ganglion) 


THE  CRANIAL  NERVES 


283 


sends  a dorsal  root  to  the  hypoglossus.  In  human  embryos,  Froriep’s  ganglion  may  be 
present  as  a rudimentary  structure  (Figs.  282  and  286),  or  it  may  be  absent  and  the  gang- 
lion of  the  first  cervical  nerve  may  also  degenerate  and  disappear.  In  pig  embryos  there 
are  one  to  four  accessory  ganglia  (including  Froriep’s)  from  which  dorsal  roots  extend  to 
the  root  fascicles  of  the  hypoglossal  nerve  (Fig.  391). 

(C)  The  Visceral  Mixed  Nerves 

The  motor  roots  of  this  group  arise  in  a lateral  series,  distinct  from  the 
dorsal  and  ventral  roots  already  described  (Figs.  256  and  278).  The 
trigeminal  nerve  contains  not  only  visceral  fibers  but  numerous  somatic 
sensory  neurons  which  supply  the  integument  of  the  head  and  face.  The 
facial,  glossopharyngeal,  and  vagus  nerves  are  essentially  visceral  in 
function.  Their  sensory  fibers  innervate  the  sense  organs  of  the  branchial 

Gang,  jiigulare  N.  10 


arches  and  viscera.  A few  somatic  sensory  fibers,  having  the  same  origin 
and  course  in  the  myelencephalon,  pass  to  the  adjacent  integument. 

5.  The  Trigeminal  Nerve  is  chiefly  sensory.  Its  large  semilunar 
ganglion,  a derivative  of  the  ganglion  crest,  arises  at  the  very  beginning  of 
the  hind  brain  (Fig.  280).  Centrally  directed  processes  form  the  large 
sensory  root  that  enters  the  wall  of  the  hind-brain  at  the  level  of  the 
pontine  flexure  (Fig.  282).  These  fibers  fork,  and  then  course  cranially 
and  caudally  in  the  alar  plate  of  the  myelencephalon;  the  caudal  fibers 
constitute  the  descending  spinal  tract  of  the  trigeminal  nerve  (Fig.  287). 
The  peripheral  processes  separate  into  three  large  divisions,  the  ophthalmic. 


284 


THE  PERIPHERAL  NERVOUS  SYSTEM 


maxillary,  and  mandibular  nerves,  and  supply  the  integument  of  the  head 
and  face  and  the  epithelium  of  the  mouth  and  tongue. 

The  motor  fibers  of  the  trigeminal  nerve  arise  largely  from  a dorsal 
motor  nucleus  that  lies  opjiosite  the  point  at  which  the  sensory  fibers  enter 
the  brain  wall  (Fig.  287).  In  the  embryo,  these  fibers  emerge  as  a sepa- 
rate motor  root,  course  along  the  mesial  side  of  the  semilunar  ganglion,  and, 
as  a distinct  trunk,  supply  the  premuscle  masses  which  later  form  the 
muscles  of  mastication.  From  the  chief  motor  nucleus,  a line  of  cells, 
extending  cranially  into  the  mesencephalon,  constitutes  a second  source  of 
origin  for  motor  fibers.  In  the  adult,  the  motor  fibers  form  a part  of  the 
mandibular  division  of  the  nerve. 


Funic. 

posterior 


Nucl.  n.  hypoglossl 


Tractus  solltarlua  Niid.  mot..i-.  n.  li-leemliil 

! Onnf.  radlcls  n.X  ; N.  trochlem  ls . - 


It.  posterior 


N. vagus 


Fig.  287  — Reconstruction  of  the  nuclei  of  origin  and  termination  of  the  cranial  nerves  in  a 
10  mm.  human  embryo  (Streeter).  X 30.  The  somatic  motor  nuclei  are  colored  red. 


7.  The  Facial  Nerve  is  composed  for  the  most  part  of  efferent  fibers 
that  supply  the  facial  muscles  of  expression.  In  10  mm.  embryos  these  ■ 
fibers  arise  from  a cluster  of  neuroblasts  in  the  basal  plate  of  the  myelen-  - 7 
cephalon,  located  beneath  the  third  neuromere  (Fig.  287).  The  fibers-- 
pass  laterally,  and  emerge  just  mesial  to  the  acoustic  ganglion.  The  motor 
trunk  then  continues  caudally  and  is  lost  in  the  tissue  of  the  hyoid  bran- 
chial arch,  tissue  which  later  gives  rise  to  the  muscles  of  expression  (Fig. 
282).  The  sensory  fibers  of  the  facial  nerve  arise  from  the  eells  of  the 
geniculate  ganglion,  which  Bartelmez  (1922)  asserts  is  a derivative  of  the 
brain  wall  rather  than  of  the  ganglion  crest.  This  ganglion  is  present  in 
7 mm.  embryos  (Fig.  280),  located  cranial  to  the  acoustic  ganglion.  The 


THE  CRANIAL  NERVES 


0 


centrally  directed  processes  of  the  geniculate  ganglion  enter  the  alar  plate 
and  form  part  of  the  solitary  tract.  Some  peripheral  fibers  course  with 
motor  fibers  in  the  chorda  tympani,  join  the  mandibular  branch  of  the  tri- 
geminal nerve,  and  end  in  the  sense  organs  of  the  tongue.  Other  sensory 
fibers  form  later  the  great  superficial  petrosal  nerve,  which  extends  to  the 
spheno-palatine  ganglion. 

The  motor  fibers  of  the  facialis  at  first  grow  straight  laterad,  passing 
cranial  to  the  nucleus  of  the  abducens.  The  nuclei  of  the  two  nerves  later 
shift  their  positions,  that  of  the  facial  nerve  moving  caudad  and  laterad, 
while  the  nucleus  of  the  abducens  shifts  cephalad.  x\s  a result,  the  motor 
root  of  the  facial  nerve  bends  around  the  nucleus  of  the  abducens,  produc- 
ing the  genu,  or  knee,  of  the  former.  Together,  they  produce  the  rounded 
eminence  in  the  floor  of  the  fourth  ventricle  known  as  the  facial  colliculiis. 

g.  The  Glossopharyngeal  Nerve  takes  its  superficial  origin  just  caudal 
to  the  otic  vesicle  (Figs.  280,  286  and  288).  Its  few  motor  fibers  arise 
from  neuroblasts  in  the  basal  plate  beneath  the  fifth  neuromeric  groove. 
These  neuroblasts  form  part  of  the  nucleus  amhiguiis,  a nucleus  of  origin 
which  the  glossopharyngeal  shares  with  the  vagus  (Fig.  287).  The  motor 
fibers  course  laterally  beneath  the  spinal  tract  of  the  trigeminal  nerve 
and  emerge  to  form  the  trunk  of  the  nerve.  These  fibers  later  supply 
the  muscles  of  the  pharynx  derived  from  the  third  branchial  arch. 

The  sensory  fibers  of  the  glossopharyngeal  nerve  arise  from  two 
ganglia,  the  su perior,  or  root  ganglion,  and  the  petrosal,  or  trunk  ganglion 
(Figs.  282  and  288).  These  fibers  constitute  the  greater  part  of  the  nerve, 
and  divide  peripherally  to  form  the  tympanic  and  lingual  rami  to  the  second 
and  third  branchial  arches.  Centrally,  the  sensory  fibers  enter  the  alar 
plate  of  the  myelencephalon  and  join  similar  fibers  of  the  facial  nerve 
coursing  caudally  in  the  solitary  tract. 

10,  II.  The  Vagus  and  Spinal  Accessory. — The  vagus,  like  the  hypo- 
glossus,  is  composite.  It  represents  the  union  of  several  nerves  which 
supply  the  branchial  arches  of  aquatic  vertebrates  (Figs.  282  and  288). 
The  more  caudal  fascicles  of  motor  fibers  take  their  origin  in  the  lateral 
gray  column  of  the  cervical  cord,  as  far  back  as  the  fourth  cervical  segment. 
These  fibers  emerge  laterally,  and,  as  the  spinal  accessory  trunk  (in  anatomy 
a distinct  nerve),  course  cephalad  along  the  line  of  the  neural  crest  (Figs. 
280,  282  and  288).  Other  motor  fibers  originate  from  the  neuroblasts 
of  the  nucleus  ambiguus  of  the  myelencephalon  (Fig.  287).  Still  others 
arise  from  a dorsal  motor  nucleus  that  lies  median  in  position.  The  fibers 
from  these  two  sources  emerge  laterally  as  separate  fascicles  and  join  the 
fibers  of  the  spinal  accessory  in  the  trunk  of  the  vagus  nerve.  The 
accessory  fibers  soon  leave  the  trunk  of  the  vagus  and  are  distributed 
laterally  and  caudally  to  the  vdsceral  premuscle  masses  which  later  form 


286 


THE  PERIPHERAL  NERVOUS  SYSTEM 


the  sterno-mastoid  and  trapezius  muscles  of  the  shoulder  (Fig.  282). 
Other  motor  fibers  of  the  vagus  supply  muscle  fibers  of  the  pharynx  and 
larynx. 

As  the  vagus  is  a composite  nerve,  it  has  several  root  ganglia  which 
arise  as  enlargements  along  the  course  of  the  ganglion  crest  (Figs.  282 


Gan g.  j ugulare  N . lo.  Accessory  root  ganglia 


Fig.  288. — The  peripheral  nerves  in  the  occipital  region  of  an  18  mm.  human  embryo  (Streeter). 

X 17- 

and  288).  The  more  cranial  of  these  is  the  jugular  ganglion.  The  others, 
termed  accessory  ganglia,  are  vestigial  structures  and  not  segmentally 
arranged.  In  addition  to  the  root  ganglia  of  the  vagus,  there  is  the  nodose 
ganglion  of  the  trunk  (Fig.  288).  The  trunk  ganglia  of  both  the  vagus 
and  glossopharyngeal  nerves  are  believed  to  be  derivatives  of  the  ganglion 


THE  SYMPATHETIC  NERVOUS  SYSTEM 


287 


crest,  their  cells  migrating  ventrad  in  early  stages.  The  central  processes 
from  the  neuroblasts  of  the  vagus  ganglia  enter  the  wall  of  the  myelen- 
cephalon,  turn  caudad,  and,  with  the  sensory  fibers  of  the  facial  and  glosso- 
pharyngeal nerves,  complete  the  solitary  tract.  The  peripheral  processes 
of  the  ganglion  cells  form  the  greater  part  of  the  vagus  trunks  after  the 
separation  from  it  of  the  spinal  accessory  fibers. 

Placodes. — In  aquatic  vertebrates,  special  somatic  sensory  fibers  from  the  lateral  line 
organs  join  the  facial,  glossopharyngeal,  and  vagus  nerves,  and  their  ganglion  cells  form 
parts  of  the  geniculate,  petrosal,  and  nodose  ganglia.  In  human  embryos,  the  organs  of  the 
lateral  line  are  represented  by  ectodermal  thickenings,  or  placodes,  which  occur  tempo- 
rarily over  these  ganglia.  The  placode  of  the  hyoid  arch  is  said  to  contribute  cells  to  the 
substance  of  the  geniculate  ganglion,  and  possibly  the  ganglia  of  the  ninth  and  tenth  nerves 
receive  similar  additions  (Bartelmez,  1922). 

Neurobiotaxis. — The  positions  of  the  motor  nuclei  vary  widely  in  the  several  verte- 
brate groups.  This  is  because  the  cell  bodies  of  motor  neurons  migrate  during  develop- 
ment toward  the  centers  from  which  their  principal  afferent  impulses  proceed.  Such  a 
re.sponse  to  some  unknown  attractive  force  is  called  neurobiotaxis. 

THE  SYMPATHETIC  NERVOUS  SYSTEM 

The  sympathetic  nervous  system  is  composed  of  a series  of  ganglia 
and  peripheral  nerves,  the  fibers  of  which  supply  gland  cells  and  the 
cardiac-  and  smooth  muscle  fibers  of  the  viscera  and  blood  vessels.  The 
nerve  cells  are  of  the  multipolar  ganglion  type  and  their  axons  remain 
unmyelinated. 

Sympathetic  ganglia  arise  from  cells  of  the  ganglion  crest  (and  the 
neural  tube),  which,  at  10  mm.,  migrate  distally  along  the  nerve  roots 
and  accumulate  in  masses  dorso-lateral  to  the  aorta  (Figs.  281  and  406). 
In  the  region  of  the  trunk,  these  paired,  segmental  clusters  unite  from 
segment  to  segment  to  form  longitudinal  cords,  which,  at  10  mm.,  are 
converted  into  nerve  fibers  that  thereafter  link  the  ganglia  in  a commis- 
sural manner  (Fig.  289).  The  resultant  ganglionated  cords  are  the  sym- 
pathetic trunks. 

Root  fibers  from  the  cerebro-spinal  nerves  pass  into  the  adjacent 
ganglia  of  the  sympathetic  trunks  (Figs.  289  and  409).  Some  are  efferent 
and  terminate  about  the  ganglion  cells,  whence  their  impulses  are  relayed 
by  unmyelinated  sympathetic  neurons  to  their  destination  (Fig.  281). 
Others  are  afferent,  bringing  visceral  sensory  impulses  directly  from  the 
viscera  to  the  spinal  ganglia  and  central  nervous  system.  Both  fiber 
types  acquire  myelin  sheaths  and  so  constitute  the  white  communicating 
rami.  Unmyelinated  sympathetic  fibers  also  grow  back  into  the  spinal 
nerves  by  separate  gray  communicating  rami.  These  are  efferent  in  func- 
tion and  are  distributed  with  the  spinal  nerves. 

In  addition  to  the  primary  ganglia  of  the  paired  sympathetic  trunks, 
there  are  other  more  peripheral  ones,  known  as  collateral  ganglia,  belonging 


288 


THE  PERIPHERAL  NERVOUS  SYSTEM 


to  the  great  prevertebral  plexuses,  such  as  the  cardiac,  coeliac,  and  hypo- 
gastric (Fig.  289).  Still  further  distad,  are  the  terminal  ganglia,  located 
near  or  even  within  the  structures  they  innervate;  this  group  includes  the 
ciliary  and  cardiac  ganglia  and  the  small  ganglion  masses  of  the  myenteric 
and  submucous  plexuses.  Each  cell  in  these  several  types  of  ganglion  is  in 
direct  relation  to  the  axon  of  a cerebro-spinal  cell,  so  that  every  sympathe- 


Fig.  289. — The  sympathetic  system  of  a 16  mm.  human  embryo  (Streeter).  X 7.  The 
ganglionated  trunk  is  heavily  shaded,  cil.,  Ciliary  ganglion;  coe.,  cceliac  artery  and  plexus; 
lit.,  heart  and  cardiac  plexus;  ot.,  otic  ganglion;  pet.,  petrosal  ganglion;  s-vi.,  submaxillary 
ganglion;  sph-p.,  spheno-jmlatine  ganglion. 

tic  neuron  forms  a terminal  link  in  a chain  whose  first  link  is  a neuron 
belonging  to  the  central  nervous  system. 

The  ganglion  cells  of  the  prevertebral  plexuses  originate,  in  embryos 
of  7 mm.,  like  those  of  the  sympathetic  trunks,  and  differ  only  in  migrating 
greater  distances.  The  terminal  ganglia  related  to  the  cardiac,  pulmonary, 
and  upper  enteric  plexuses  arise  at  about  the  same  time  from  cells  of 
cerebro-spinal  origin  which  advance  peripherally  along  the  vagus  nerves. 


THE  CHROMAFFIN  BODIES  AND  SUPRARENAL  GLAND 


289 


The  adult  cervical  sympathetic  ganglia  represent  fusions  between  the 
primitive  ganglionic  masses  of  this  region  (Figs.  288  and  289).  The 
sympathetic  ganglia  related  to  the  brain  are  from  the  first  unsegmental. 
They  are  derived  chiefly  from  the  primitive  semilunar  ganglia,  although  the 
brain  wall  and  the  geniculate  and  petrosal  ganglia  also  contribute  (Fig.  289). 


THE  CHROMAFFIN  BODIES  AND  SUPRARENAL  GLAND 


Certain  cells  of  the  sympathetic  ganglia  are  transformed  into  peculiar 
glands,  rather  than  into  neurones.  The  internal  secretion  formed  by 
these  elements  causes  them  to  stain  brown  when  treated  with  chrome  salts — 
hence  the  designation,  chromaffin  cells.  Cells  of  this  type,  derived  from  the 


Fig.  290. — Section  through  a chromaffin  body  in  a human  fetus  of  ten  weeks  (after  Kohn). 

X 450. 


ganglionated  cord  of  the  sympathetic  system,  give  rise  to  structures 
known  as  chromaffin  bodies.  Chromaffin  derivatives  of  the  coeliac  plexus, 
together  with  mesenchymal  tissue,  also  form  the  suprarenal  gland. 

The  chromaffin  bodies  of  the  ganglionated  cords  are  rounded,  cellular 
masses  partly  embedded  in  the  dorsal  surfaces  of  the  ganglia,  and  so 
termed  paraganglia  (Fig.  290).  They  appear  during  the  third  month,  and, 
at  birth,  may  attain  a diameter  of  i to  1.5  mm.  In  number  they  vary 
from  one  to  several  for  each  ganglion.  Similar  chromaffin  bodies  may 
occur  in  all  the  larger  sympathetic  plexuses.  The  largest,  found  in  the 
abdominal  sympathetic  plexuses,  are  the  aortic  chromaffin  bodies{  of  Zucker- 


2QO 


THE  PERIPHERAL  NERVOUS  SYSTEM 


kandl).  These  occur  in  embryos  of  seven  weeks,  on  either  side  of  the 
inferior  mesenteric  artery,  ventral  to  the  aorta  and  mesial  to  the  meta- 
nejihros.  At  birth,  they  attain  a length  of  about  i cm.  and  are  composed 
of  cords  of  chromaffin  cells  intermingled  with  strands  of  connective  tissue, 
the  whole  being  surrounded  by  a connective-tissue  capsule.  After  birth 
the  chromaffin  bodies  degenerate,  but  do  not  disappear  entirely.  Asso- 
ciated with  the  intercarotid  sympathetic  plexus  is  a highly  vascular 
chromaffin  organ  known  as  the  carotid  body.  Its  analge  has  been  first 
observed  in  emliryos  of  seven  weeks. 

d'he  Suprarenal  Gland  has  a double  origin.  The  cortex  is  derived 
from  mesoderm,  the  medulla  from  chromaffin  tissue.  In  an  embryo  of  6 


Capsule 


Fig.  291. — Section  through  the  right  suprarenal  gland  of  a 16  mm.  human  embryo  (Bryce). 
*,  Invading  groups  of  chromaffin  cells. 


mm.,  the  anlage  of  the  cortex  begins  to  form  from  ingrowing  buds  of  the 
peritoneal  mesothelium;  this  proliferation  occurs  on  each  side  of  the 
mesentery,  near  its  root.  At  about  9 mm.  the  paired  glands  are  definite 
organs  and  their  vascular  structure  is  evident  (Fig.  114).  The  anlages  of 
the  suprarenals  early  project  from  the  dorsal  wall  of  the  coelom,  between 
the  mesonephros  and  mesentery;  here  they  become  relatively  huge  organs 
(Figs.  145,  154  and  155).  The  differentiation  of  the  cortex  into  its  three 


THE  CHROMAFFIN  BODIES  AND  SUPRARENAL  GLAND  2QI 

characteristic  layers  is  not  completed  until  between  the  second  and  third 
years.  The  inner  reticular  zone  is  formed  first,  the  fasciculate  zone  next, 
and  finally  the  glomerular  zone  appears  during  the  third  month. 

The  chromaffin  cells  of  the  medulla  are  derived  from  the  coeliac  plexus 
of  the  sympathetic  system.  In  embryos  of  seven  weeks,  whien  the  cortex 
is  already  prominent,  masses  of  these  cells  begin  to  migrate  from  the 
median  side  of  the  suprarenal  anlage  to  a central  position  (Fig.  291). 
Such  penetration  probably  continues  until  after  birth.  The  primitive 
chromaffin  cells  are  small  and  stain  intensely. 

Anomalies. — Portions  of  the  suprarenal  anlage  separate  frequently  from  the  parent 
gland  and  form  accessory  suprarcnals.  As  a rule,  such  accessory  glands  are  composed 
only  of  cortical  substance;  they  may  migrate  some  distance  from  their  original  position, 
accompanying  the  genital  glands.  In  fishes,  the  cortex  and  medulla  occur  normally  as 
separate  organs. 


CHAPTER  XV 


THE  SENSE  ORGANS 


The  sense  cells  of  primitive  animals,  such  as  worms,  are  ectodermal 
in  origin  and  position.  Only  those  of  the  vertebrate  olfactory  organ  have 
retained  this  primitive  positional  relation,  although  the  germ-layer  origin 
is  unchanged.  During  ]jhylogenesis  the  cell-bodies  of  all  other  such  pri- 
mary sensory  neurones,  except  smell,  migrated  inward  to  form  the  dorsal 
ganglion  (Parker),  hence  their  peripheral  processes  either  end  freely  in 
the  epithelium,  are  associated  with  various  sensory  corpuscles,  or  appro- 
priate new  cells  to  serve  as  sensory  receiptors  (taste;  hearing). 

Among  the  sense  organs  are  receptive  elements  of  general  sensibility 
which  Ipelong  to  the  integument,  muscles,  tendons,  and  viscera;  these 
mediate  such  sensations  as  touch,  pressure,  muscle  and  tendon  sensibility, 
temperature,  and  pain.  Other  organs,  of  a special  sensory  nature,  are 
responsible  for  the  sensations  of  taste,  smell,  vision,  and  hearing.  Each  is 
tuned  to  a specific  and  exclusive  kind  of  stimulus.  The  organs  of  smell, 
vision,  and  hearing  are  distance  receptors,  in  contrast  to  all  others  which 
collect  information  from  the  organism  itself  and  especially  from  its  integu- 
ment. The  apparatus  for  smell  and  taste  consists  of  little  more  than  the  ' 
special  sensory  cells  alone,  whereas  the  eye  and  ear  possess  elaborate 
accessory  mechanisms  for  receiving  the  external  stimulus  and  converting 
it  into  a form  suitable  to  affect  the  sensory  cells  proper. 

GENERAL  SENSORY  ORGANS 

Free  nerve  terminations  form  the  great  majority  of  all  the  general 
sensory  organs.  When  no  sensory  corpuscle  is  developed,  the  neuro- 
fibrils of  the  sensory  nerve  fibers  separate  and  end  among  the  cells  of  the 
epithelia. 

Laniellated  corpuscles  first  arise  during  the  fifth  month  as  masses  of 
mesodermal  cells  clustered  around  a nerve  termination.  The  cells  multi- 
ply, flatten,  and  give  rise  to  concentric  lamellce.  In  the  cat,  these  corpuscles 
increase  in  number  by  budding. 

Tactile  corpuscles  are  said  to  develop  from  mesenchymal  cells  and  | 
branching  nerve  fibrils  during  the  first  six  months  after  birth. 


THE  GUSTATORY  ORGAN 

In  fetuses  of  three  months,  thickenings  of  the  lingual  epithelium 
represent  the  future  taste  buds.  The  parent  tissue  is  quite  clearly  ento- 

2Q2 


f lire.  ^ 77^ 


Tic  cl  €■  ^ ^ y'  n- i 


THE  NOSE 


293 


derm,  yet  an  ectodermal  origin  is  sometimes  asserted.  The  cells  of  an 
anlage  lengthen  and  extend  to  the  surface  of  the  epithelium.  The  ovoid 
mass  then  differentiates  into  sensory  taste  cells,  with  modified  cuticular 
tips,  and  into  supporting  cells.  Taste  buds  are  supplied  by  nerve  fibers  of 
the  seventh,  ninth,  and  tenth  cranial  nerves;  the  fibers  branch  and  end  in 
contact  with  the  periphery  of  the  taste  cells. 

Between  the  fifth  and  seventh  fetal  months,  taste  buds  are  more 
widely  distributed  than  in  the  adult.  They  are  found  in  the  walls  of  the 


D 


E 


Nasal  fossa 


process 


Telencephalon 


Lai.  nasal  process 
Med.  nasal  process 
Maxillary  process 


Fig.  292. — Sections  through  the  olfactory  anlages  of  human  embryos  (adapted  by  Prentiss). 

X about  15.  A,  4.9  mm.;  B,  6.5  mm.;  C,  8.8  mm.;  D and  E,  10  mm. 

vallate,  fungiform,  and  foliate  papillae  of  the  tongue,  on  the  under  surface 
of  the  tongue,  on  both  surfaces  of  the  epiglottis,  on  the  palatine  tonsils 
and  arches,  and  on  the  soft  palate.  After  birth,  many  taste  buds  degen- 
erate, only  those  on  the  lateral  walls  of  the  vallate  and  foliate  papillae,  on 
a few  fungiform  papillae,  and  on  the  laryngeal  surface  of  the  epiglottis 
persisting.  The  development  of  the  several  papillae  has  been  described  in 
an  earlier  chapter  (p.  96). 

THE  NOSE 

The  olfactory  epithelium  arises  in  embryos  of  about  4 mm.  as  paired 
ectodermal  thickenings,  on  the  ventro-lateral  sides  of  the  head  (Fig.  292 


2Q4 


THE  SENSE  ORGANS 


y4).  S|)ecimens  8 mm.  long  show  these  placodes  depressed  into  olfactory 
pits,  or  fosscc,  about  which  the  nose  develops  (Figs.  184  and  292  B,  C). 

d'he  detailed  history  of  the  olfactory  organ  is  associated  with  that  of 
the  face.  It  will  be  remembered  (p.  77)  that  each  first  branchial  arch 
forks  into  a maxillary  and  mandibular  process.  Dorsal  to  the  mouth  is 
the  fronto-nasal  process  of  the  head,  lateral  to  it  the  maxillary  processes, 
and  ventral  to  it  are  the  mandibular  processes  (Fig.  3 6g).  With  the  appear- 
ance of  the  nasal  pits,  the  lower  part  of  the  fronto-nasal  process  necessarily 


Nasal  septum 


Ext.  naris 
Lat.  nasal 
process 
M cd.  nasal 
process 
Maxillary 
process 
Mandible 


Oral  cavity 


.1 


Ext.  naris 


Lat. 
process 


Maxillary 

process 


Med.  nasal  process  Oral  cavity 

B 


Fig.  29-5, — Stages  in  the  development  of  the  human  face.  A,  10.5  mm.  (Peter);  B,  11.3  mm. 

(Rabl). 


is  sufulivided  into  paired  lateral  and  median  ttasal  processes  (Fig.  293  A) 
The  nasal  depressions  are  at  first  grooves,  each  bounded  mesially  by 
the  median  nasal  jtrocess  and  laterally  by  the  lateral  nasal  and  maxillary 
processes.  The  prompt  fusion  of  the  maxillary  (trocesses  with  the  median 
nasal  processes  converts  the  nasal  grooves  into  blind  pits,  opening  by 
external  nares  (Fig.  293  A),  and  separated  from  the  mouth  cavity  by 
ectodermal  plates  (Fig.  293  D,  E).  The  mutual  union  of  the  median  and 
lateral  nasal  processes  reduces  still  further  the  size  of  the  external  nares 
(Fig.  293  B).  The  epithelial  plates  which  separate  the  nasal  fossse  from 
the  primitive  mouth  cavity  become  thin,  membranous  structures  caudally, 
and,  rupturing,  produce  two  internal  nasal  openings,  the  primitive  choana: 
(Fig.  74).  The  front  part  of  the  plate  is  invaded  by  mesoderm,  thereby 
forming  the  primitive  palate  (Fig.  292  D) ; the  latter  becomes  the  lip 
and  the  premaxillary  palate.  The  nasal  fossae  now  open  externally  through 


\ 

: 

i 

'! 


THE  NOSE 


295 


the  external  nares,  and  internally  into  the  roof  of  the  mouth  cavity  through 
the  primitive  choanae. 

Coincident  with  these  changes,  the  median  frontal  process  has  become 
relatively  narrower,  and  that  portion  of  it  between  the  nasal  fossae  serves  as 
the  nasal  septum  (Fig.  293).  By  the  development  and  fusion  of  the 
palate  anlages  (p.  87),  the  dorsal  portion  of  the  mouth  cavity  is  presently 
partitioned  off  as  the  nasal  passages  (Figs.  294  and  295).  The  passages 
of  the  two  sides  for  a time  communicate  through  the  space  between  the 


Olfactory  epithelium 


Voniero-nasal  organ 


Inferior 


Palatine  process 


Dental 


Cartilage  of  nasal 
septum 


of  vomer 0- 
Naso-lacrimal  duct 


Tongue 


Meckel’s  cartilage 


Fig.  294. — Section  through  the  nose  and  mouth  of  a human  embryo  of  seven  weeks  (Prentiss). 

X 30. 


hard  palate  and  the  nasal  septum  (Fig.  294),  but  later,  the  ventral  border 
of  the  septum  fuses  with  the  hard  palate  and  separates  them  completely 
(Fig.  295).  The  definitive  nasal  passages  thus  consist  of  the  primitive 
nasal  fossae  plus  a portion  of  the  primitive  mouth  cavity  which  has  been 
appropriated  secondarily  by  the  development  of  the  hard  palate.  Their 
internal  opening  into  the  pharynx  is  by  .secondar}",  permanent  choance. 
From  the  second  to  the  sixth  month  the  external  nares  are  closed  by 
epithelial  plugs. 

The  lining  of  the  upper  part  of  the  primitive  fossae  is  transformed 
into  olfactory  epithelium  (Figs.  294  and  295).  Many  of  its  ciliated  cells 
become  elongate  sensory  elements  with  olfactory  nerve  fibers  growing 
from  their  basal  ends  (Fig.  283).  The  rest  of  the  nasal  epithelium,  origi- 


2q6 


THE  SENSE  ORGANS 


nally  a part  of  the  mouth  cavity  and  now  respiratory  in  function,  covers  the 
conchas  and  lines  the  vomero-nasal  organ,  ethmoidal  cells,  and  paranasal 
sinuses  (Fig.  296). 


Fig.  295. — Section  through  the  nasal  passages  of  a three-months’  fetus  (Prentiss).  X 14. 

The  Vomero-nasal  Organs  (oj  Jacobson)  are  rudimentary  epithelial 
structures  which  first  appear  in  9 mm.  embryos  as  paired  grooves  on  the 
median  walls  of  the  nasal  fossse  (Fig.  292  C,  E).  The  grooves  deepen  and 


Fig.  296. — Right  nasal  passage  of  a fetus  at  term  (Killian-Prentiss).  I,  Maxillo-turbinal; 
11-VI,  ethmo-turbinals.  The  slight  elevation  at  the  left  of  I and  II  is  the  naso-turbinal. 

close  caudally  to  form  tubular  sacs,  opening  toward  the  front  of  the  nasal 
septum  (Fig.  294).  Nerve  fibers,  arising  from  the  epithelial  cells  of  the 
organ,  join  the  olfactory  nerve,  and  other  fibers  from  the  terminal  nerve 


THE  EYE 


297 


are  received  into  it.  During  the  sixth  month  the  vomero-nasal  organ 
attains  a length  of  4 mm.  and  special  cartilages  are  developed  early  for  its 
support  (Fig.  294).  In  late  fetal  stages  it  often  degenerates,  but  may 
persist  in  the  adult.  This  organ  is  not  functional  in  man,  but  in  many 
animals  it  evidently  constitutes  a special  olfactory  apparatus. 

The  human  Concha;  are  poorly  developed.  They  include  several 
elevated  folds  on  the  lateral  and  median  walls  of  the  nasal  foss?e.  The 
maxillo-tiirhinal  is  developed  first,  followed  by  five  ethmo-tiirhinals  arranged 
in  order  of  decreasing  size  (Figs.  294  to  296).  According  to  Peter,  the 
ethmo-turbinals  arise  on  the  median  walls  of  the  fossae,  and,  by  a process 
of  unequal  growth,  are  transferred  to  the  lateral  walls.  The  naso-turbinal 
is  very  rudimentary  and  appears  as  a slight  elevation  dorsal  and  cranial 
to  the  maxillo-turbinal  (Fig.  296).  In  adult  anatomy,  the  inferior  concha 
forms  from  the  maxillo-turbinal  (I),  the  middle  concha  from  the  first  ethmo- 
turbinal  {II),  and  the  superior  concha  from  the  second  and  third  ethmo- 
turbinals  {III,  ID).  The  naso-turbinal  becomes  the  agger  nasi. 

In  communication  with  the  nasal  cavity  are  several  irregular  chambers, 
known  collectively  as  the  paranasal  sinuses.  The  ethmoidal  cells  develop 
in  the  grooves  between  the  ethmo-turbinals.  During  the  third  month  the 
maxillary  sinus  begins  to  evaginate  from  the  groove  between  I and  II 
(Fig.  296),  and,  after  birth,  the  superior  portion  of  the  same  furrow  gives 
rise  to  the  frontal  sinus.  The  caudal  end  of  each  nasal  fossa  is  set  aside 
during  the  third  month  as  a sphenoidal  sinus  which  secondarily  invades 
the  sphenoid  bone  to  accommodate  its  increasing  size.  These  cells  and 
sinuses  represent  excavations  of  bone  which  become  lined  wdth  simul- 
taneously advancing  epithelium. 


THE  EYE 

The  eye  is  a derivative  of  the  fore-brain.  In  embryos  of  2.5  mm.,  eY^en 
before  this  region  of  the  neural  groove  closes,  the  evaginated  anlages  are 
recognizable  (Fig.  305).  Soon,  at  4 mm.,  distinct  optic  vesicles  are  attached 
to  the  brain  wall  by  hollow  optic  stalks  (Figs.  251  and  297),  and  this  condi- 
tion is  followed  promptly  by  the  stage  of  the  optic  cup  in  which  there  is 
an  invagination  of  the  distal  wall  of  the  vesicle  to  form  a double-layered 
crater  (Figs.  252  A,  297  B-D,  and  299).  The  optic  cup  is  destined  to 
become  the  retina,  or  the  essential  sensory  epithelium  of  the  eye,  and  the 
optic  nerve.  Meanwhile,  the  surface  ectoderm,  overlying  the  optic  vesicle, 
thickens  into  a placode  (Fig.  297  B)  that  presently  pockets  inward  to 
produce  the  leiis  vesicle,  or  anlage  of  the  lens  (Fig.  297  C,  D).  The  acces- 
sory vascular  and  fibrous  coats  differentiate  from  the  adjacent  mesoderm 
(Fig.  299).  With  this  introductory  explanation  for  a background,  the 
details  of  development  may  now'  be  set  forth. 


298 


THE  SENSE  ORGANS 


The  experiments  of  Stockard  suggest  that  the  earliest  optic  anlage  may  be  a median 
area  of  the  fore-brain  wall  which  separates  into  two  placodes  that  migrate  laterad  to  the 
positions  where  they  are  usually  first  recognized. 

The  invagination  of  the  optic  vesicle  is  a self-governed  process.  On  the  contrary, 
contact  of  the  optic  vesicle  with  the  overlying  ectoderm  stimulates  the  latter  to  lens  forma- 
tion, even  in  regions  that  normally  never  differentiate  a lens  (Lewis,  1907).  It  is  possible, 
however,  for  a lens  to  arise  independently  of  this  contact  stimulus  (Stockard,  1910). 


A B 


Fig.  297. — Stages  in  the  early  development  of  the  human  eye  (Keibel  and  Elze-Prentiss). 
X about  23.  /I,  B,  4 mm.;  C,  5 mm.;  D,  6.3  mm. 


Fig.  298. — The  optic  stalk,  cup  and  lens  of  a human  embryo  of  12.5  mm.  (after  Hochstetter)- 
X 90.  The  chorioid  fissure  has  closed  along  the  optic  stalk. 

Differentiation  of  the  Optic  Cup. — From  the  first,  the  optic  cup  is 
imperfect,  inasmuch  as  its  region  of  invagination  extends  also  ventrally 
along  the  optic  stalk  (Fig.  252  A).  This  produces  a defect  in  the  rim  of 


THE  EYE 


299 


the  cup,  continuous  with  a furrow-like  groove  of  the  stalk  known  as  the 
chorioid  fissure  (Fig.  298).  As  a necessary  result,  both  the  inner  and  outer 
layer  of  the  optic  cup  are  continued  into  the  stalk  (Fig.  299).  During 
the  sixth  or  seventh  week  the  lips  of  the  chorioid  fissure  close,  so  that  the 
distal  rim  of  the  optic  cup  then  forms  a complete  circle. 

The  development  of  the  optic  cup  obliterates  the  cavity  of  the  primitive 
spherical  vesicle  (Figs.  297  and  299).  Its  two  component  layers  lie  in 
apposition  and  transform  into  the  epithelial  retina.  The  outer,  thinner 
layer  becomes  the  pigment  layer.  Pigment  granules  appear  in  its  cells  in 
embryos  of  7 mm.  and  the  pigmentation  is  soon  dense  (Fig.  303). 

Mesenchyme  Lens  vesicle  Vitreous  body  Optic  stalk  Optic  recess  of  brain 


Epithelium  of  cornea  Pigment  layer  of  retina  Nervous  layer  of  retina 

Fig.  299. — Section  through  the  optic  cup,  stalk  and  lens  of  a 10  mm.  human  embryo  (Prentiss). 

X 100. 

The  inner,  thicker  layer  of  the  optic  cup  is  the  retinal  layer  proper. 
In  it  may  be  recognized  the  pars  cceca,  a non-nervous  zone  bordering  the 
rim,  and  the  pars  optica,  or  the  true  nervous  portion.  The  line  of  demarca- 
tion between  these  two  regions  is  a scalloped  circle,  the  ora  serrata.  By  the 
development  of  the  mesodermal  ciliary  bodies,  the  pars  caeca  is  subdivided 
into  a pars  ciliaris  and  pars  iridica.  The  former,  with  a corresponding 
zone  of  the  pigment  layer,  covers  the  ciliary  bodies.  The  pars  iridica 
blends  intimately  with  the  pigment  layer  and  beeomes  similarly  pigmented 
(Fig.  304).  It  forms  the  inner  eovering  of  the  iris. 

The  pars  optica,  or  nervous  portion  of  the  retina,  begins  to  differen- 
tiate near  the  optic  stalk  and  the  differentation  extends  peripherally.  An 
outer,  cellular  layer  (next  the  pigment  coat)  and  an  inner,  fibrous  layer 


300 


THE  SENSE  ORGANS 


may  be  distinguished  in  12  mm.  embryos  (Fig.  299).  These  correspond  to 
the  cellular  layer  (ependymal  and  mantle  zones)  and  marginal  layer  of  the 
neural  tube.  At  three  montlis,  the  retina  shows  three  strata,  large  ganglion 


limiting 

..  membrane 


•Layer  of  rod  and  cone 
cells 


Cone  cell 
Rod  cell 

Rod  cell 
Fiber  of  M idler 

A macrine  cell 
Ganglion  cell 
Optic  fibers 


'iglionic  layer 
Fibrous  layer 

■iPJ  b ■ I nternal  limiting 

A;ii.  membrane 

Fig.  300. — Section  of  the  nervous  layer  of  the  retina  from  a fetus  of  three  months  (Prentiss). 
X 440.  At  the  left  are  the  component  elements  according  to  Cajal. 


layer 
Rods  and  Cones 


Older  nuclear  layer 


— Outer  reticular  layer 


^ — Inner  nuclear  layer 

■ Inner  reticular  layer 
Ganglion  cell  layer 


Nerve  fiber  layer 
Fibers  of  Muller 


'Internal  limiting  membrane 
Fig.  301. — Section  through  the  retina  of  a seven-months’  fetus  (Prentiss).  X 440- 


cells  having  migrated  in  from  the  outer  cellular  layer  of  rods  and  cones 
(Fig.  300).  In  a fetus  of  the  seventh  month,  all  the  layers  of  the  adult 
retina  may  be  recognized  (Fig.  301).  As  in  the  wall  of  the  neural  tube, 
both  supporting  and  nervous  tissue  appear.  The  supporting  elements, 


THE  EYE 


301 


ov  fibers  of  Midler,  resemble  ependymal  cells  and  are  arranged  radially 
(Figs.  300  and  301).  Their  terminations  form  internal  and  external 
limiting  membranes.  The  outermost  neuroblasts  of  the  retina  differen- 
tiate into  rod  and  cone  cells,  the  receptive  visual  cells  of  the  retina,  which 
are  at  first  unipolar  (Fig.  301).  Next  in  position  comes  an  intermediate 
layer  of  bipolar  cells.  The  inner  stratum  of  multipolar  cells  constitutes  the 
ganglion  cell  layer;  axons  from  its  cells  form  the  nerve  fiber  layer.  These 
converge  to  the  optic  stalk,  and,  in  embryos  of  15  mm.,  grow  back  in  its 
wall  to  the  brain  (Fig.  284  N).  The  cells  of  the  optic  stalk  are  converted 

Epithelial  layer 


Capsule 

Vascular 
tunic 
of  lens 

Lens  fibers 


Fig.  302. — Section  through  the  lens  and  corneal  ectoderm  of  a 16  mni.  pig  embryo  (Prentiss). 

X 140. 

into  a scaffolding  of  neuroglia' supporting  tissue,  and  the  cavity  in  the  stalk 
is  gradually  obliterated  (Fig.  284  5).  The  optic  stalk  is  thus  transformed 
into  the  optic  nerve,  containing  a central  artery  and  vein  which  originally 
coursed  along  its  open  groove  (Fig.  303;  cf.  p.  299). 

The  Lens. — For  a short  time  the  lens  vesicle  nearly  fills  the  cavity 
of  the  optic  cup  and  is  attached  to  the  parent  ectoderm.  In  embryos  of 
8 mm.,  it  lies  free  of  both  surface  ectoderm  and  optic  cup  as  a sac  whose 
proximal  wall  is  thicker  than  the  distal  one  (Fig.  299).  The  cells  of  the 
distal  wall  remain  of  a low  columnar  type,  and  constitute  the  lens  epi- 
thelium (Fig.  302).  The  cells  of  the  inner  wall  increase  rapidly  in  height 
(Fig.  303),  and,  at  about  seven  weeks,  obliterate  the  original  cavity  (Fig. 
302).  These  cells  transform  into  lens  fibers  and  their  nuclei  degenerate. 
Toward  the  end  of  the  third  month,  the  primary  lens  fibers  attain  a length 


302 


THE  SENSE  ORGANS 


of  o.i8  mm.,  whereupon  they  cease  forming  new  fibers  by  cell  division. 
All  additional  fibers  arise  from  the  cells  of  the  epithelial  layer  at  its  equa- 
torial junction  with  the  lens-fiber  mass.  Lens  sutures  are  formed  on  the 
proximal  and  distal  faces  of  the  lens  when  the  longer,  newly  formed,  peri- 
pheral fibers  overlap  the  ends  of  the  shorter,  central  fibers  (Fig.  304). 
By  an  intricate  but  orderly  arrangement  of  fibers  these  sutures  are  later 
transformed  into  lens- stars  of  three,  and  finally  of  six  or  nine  rays.  The 
structureless  capsule  of  the  lens  is  apparently  derived  from  the  lens  cells. 
The  fetal  lens  is  spherical  and  relatively  large  (Fig.  304). 


Epithelial  layer  of  lens  Pigment  layer  of  the  retina 


Fig.  303. — Section  through  the  optic  cup  and  chorioid  fissure  of  a 12.5  mm.  human  embryo 

(Prentiss).  X 105. 


The  Vitreous  Body  and  Intraocular  Vessels. — The  space  between  the  > 
retina  and  lens  becomes  filled  with  a peculiar  hyaline  tissue,  designated  the  > 
vitreous  body  (Figs.  303  and  304).  Modern  investigations  agree  that  this 
substance  is  primarily  a product  of  the  retina,  formed  in  the  following 
way:  Processes,  probably  derived  from  the  early  supporting  cells  of  i 

Muller,  project  from  the  surface  of  the  retina  and  constitute  a fine,  '■ 
fibrillar  reticulum.  This  is  the  primitive  vitreous  (Fig.  299).  Those 
fibers  formed  by  the  pars  ciliaris  retinae  seemingly  become  the  zonula  y 
ciliaris,  or  suspensory  ligament  of  the  lens.  Whether  the  lens  itself  . 
participates  in  vitreous  development  is  disputed. 

Only  when  the  primitive  vitreous  body  is  partly  formed  does  mesen- 
chyme first  appear  within  the  optic  cup.  It  enters  with  the  central 
artery,  which,  in  embryos  of  6 mm.,  courses  along  the  gutter-like  groove  in 
the  optic  stalk,  and  extends  as  the  hyaloid  artery  through  the  chorioid 
fissure  of  the  optic  cup  toward  the  lens  (Fig.  303).  The  fate  of  this 


THE  EYE 


303 


invading  mesenchyme — whether  it  contributes  to  the  structure  of  the 
vitreous,  or  whether  it  degenerates — is  not  yet  decided  beyond  question. 

The  hyaloid  artery  and  its  accompanying  mesenchyme  vascularize 
the  back  surface  of  the  lens  (Fig.  302).  Other  vessels  from  the  peripheral 
chorioid  supply  the  front  of  the  lens  in  the  corresponding  pupillary  mem- 
brane (Fig.  304).  The  investment  as  a whole  constitutes  the  vascular 
tunic  of  the  lens.  Its  highest  development  is  attained  in  the  seventh 


1 

i 

i 


I 


Anterior  epithelium 
of  cornea 


Fusion  of  eve! ids  and 


Posterior  epithelium 
of  cornea 


Cornea 

[Lens  epithe- 
J liutn  covered 
I by  pupillary 
[ membrane 
\Pars  iridica 
] retincE  {folds 
I are  pars  cil- 
l iaris) 

Pignien  layer 
of  retina 

Pars  optica 
retina: 


Lens  fibers 


Lens  cap  ule 


Vitreous 

body 


Fig.  304. — Section  through  the  distal  half  of  the  eyeball  and  eyelids  of  a three-months’  fetus 

(Prentiss).  X 35. 


I month,  whereas  at  birth  the  tunic  has  usually  disappeared.  The  hyaloid 
i artery  also  degenerates  completely,  the  only  trace  being  the  space  of  the 
hyaloid  canal. 

The  Fibrous  and  Vascular  Coats. — AVhen  the  lens  detaches  from  the 
overlying  ectoderm,  migrant  mesenchymal  cells  fill  the  intervening  space 
(Figs.  299  and  303),  and  both  lens  and  optic  cup  become  invested  with  a 
i double  layer  of  condensed  mesenchyme.  The  outer,  more  compact 


304 


THE  SENSE  ORGANS 


sheath  is  the  anlage  of  the  fibrous  coat  which  differentiates  into  the  sclera 
and  cornea  (Fig.  304).  The  inner,  looser  sheath  will  form  the  vascular 
coat  which  includes  the  iris,  ciliary  body,  and  chorioid. 

The  tough,  fibrous  sclera  covers  the  base  and  sides  of  the  eyeball. 
It  corresponds  to  the  dura  mater  of  the  brain.  During  the  eighth  week, 
fluid-filled  clefts  appear  in  the  mesenchyme  between  the  lens  and  the 
surface  ectoderm;  these  coalesce  into  a larger  cavity,  the  anterior  chamber 
of  the  eye  (Fig.  304).  The  mesodermal  layer  then  located  in  front  of 
the  chamber,  and  continuous  with  the  sclera,  is  the  cornea  (Fig.  304). 
Externally,  it  is  covered  with  ectoderm,  and  the  whole  area  becomes  trans- 
parent at  the  end  of  the  fourth  month.  The  mesodermal  tissue  between 
the  lens  and  the  anterior  chamber  is  the  temporary  pupillary  membrane. 
The  continued  lateral  extension  of  the  anterior  chamber  presently  sepa- 
rates the  iris  from  the  cornea  (Fig.  304). 

The  inner  mesenchymal  investment,  between  the  anlage  of  the 
sclerotic  and  the  pigment  layer  of  the  retina,  acquires  a high  vascularity 
during  the  sixth  week.  Its  cells  become  stellate  and  pigmented,  so  that 
the  tissue  is  loose  and  reticulate.  This  vascular  tissue  constitutes  the 
chorioid,  in  which  course  the  chief  vessels  of  the  eye;  it  corresponds  to  the 
pia  mater  of  the  brain.  Distal  to  the  level  of  the  ora  serrata,  the  vascular 
coat  differentiates  into;  ( i)  the  vascular  folds  of  the  ciliary  bodies;  (2)  the 
smooth  fibers  of  the  ciliary  muscle;  (3)  the  stroma  of  the  iris.  The  pig- 
mented layers  of  the  iris  are  derived  both  from  the  pars  iridica  retinae  and 
from  a corresponding  zone  of  the  pigment  layer.  Of  these,  the  pigment- 
layer  cells  give  rise  to  the  pupillary  muscles  of  the  iris.  These  smooth 
muscle  fibers  are  thus  of  ectodermal  origin. 

Accessory  Apparatus. — The  Eyelids  develop  as  folds  of  the  integu- 
ment bordering  the  eyeball.  The  folds  appear  at  the  end  of  the  seventh 
week,  and  two  weeks  later  their  edges  have  met  and  fused  (Fig.  304). 
This  epidermal  union  persists  until  the  seventh  or  eighth  month.  A 
third,  rudimentary  eyelid,  corresponding  to  the  functional  nictitating 
membrane  of  lower  vertebrates,  constitutes  the  adult  plica  semilunaris. 
The  epidermis  of  the  lid  is  reflected  as  a mucous  membrane  over  the  inner 
surface,  where  it  is  known  as  the  conjunctiva ; this  in  turn  is  continuous 
with  the  conjunctival  epithelium  of  the  cornea.  The  cilia,  or  eyelashes, 
develop  like  ordinary  hairs  at  the  edges  of  the  lids  (Fig.  304);  they  are 
provided  with  both  sebaceous  glands  (of  Zeiss)  and  modified  sweat  glands 
(of  Moll).  About  30  tarsal  glands  also  arise  along  the  edge  of  each  lid; 
these  Meibomian  glands  are  sebaceous  in  nature.  The  cilia  and  small 
glands  just  mentioned  all  develop  while  the  eyelids  are  still  fused. 

The  Lacrimal  Glands  appear  during  the  ninth  week  as  approximately 
six  knobbed  ingrowths  of  the  conjunctiva.  They  lie  dorsad  near  the 


THE  EAR 


305 


external  angle  of  the  eye.  At  first  solid  epithelial  cords,  they  soon  branch 
and  acquire  lumina. 

The  Naso-lacrimal  Duct  arises  in  12  mm.  embryos  as  a ridge-like 
thickening  of  the  epithelial  lining  of  the  naso-lacrimal  groove  (Fig.  227), 
which,  it  will  be  remembered,  extends  from  the  inner  angle  of  the  eye  to 
the  primitive  olfactory  fossa.  This  thickening  becomes  cut  off,  and,  as  a 
solid  cord,  sinks  into  the  underlying  mesoderm  (Figs.  294  and  295). 
Secondary  sprouts,  growing  out  to  each  eyelid,  comprise  the  lacrimal  ducts. 

Anomalies. — Lack  of  pigment  in  the  retina  and  iris  is  usually  associated  with  general 
albinism.  A retention  of  the  pupillary  membrane  causes  congenital  atresia  of  the  pupil. 
If  the  chorioid  fissure  fails  to  close  properly,  there  results  a gaping,  and  hence  unpigmented 
defect,  or  coloboma,  in  the  iris,  ciliary  body,  or  chorioid.  In  cyclopia,  a single  median  eye 
replaces  the  usual  paired  condition.  All  intergrades  exist  from  closely  approximated, 
separate  eyes  to  perfect  unity.  The  mode  of  genesis,  whether  from  the  fusion  of  separate 
•eyes  or  from  the  inhibited  separation  of  a common  anlage  into  its  bilateral  derivatives,  is  in 
dispute.  In  cases  of  cyclopia  the  nose  is  usually  a cylindrical  proboscis,  situated  above  the 
median  eye. 

THE  EAR 

The  human  ear  consists  of  a sound-conducting  apparatus  and  a 
receptive  organ.  The  transmission  of  sound  is  the  function  of  the  exter- 

Acoustic  ganglion  Auditory  placode 


Optic  vesicle 
A 

Fig.  305. — Horizontal  sections  through  the  early  auditory  anlages  (Keibel  and  Elze-Prentiss) • 

X 30.  A,  2 mm.;  B,  4 mm. 

nal  and  middle  ears.  The  end  organ  proper  is  the  internal  ear,  with  audi- 
tory reception  residing  in  the  cochlear  duct.  Besides  an  acoustic  func- 
tion the  labyrinthine  portion  of  the  internal  ear  serves  as  an  organ  of 
equilibration. 

The  Internal  Ear. — -The  epithelium  of  the  internal  ear  is  derived  from 
the  ectoderm.  Its  anlage  appears  in  embryos  of  2 mm.  as  a thickened, 
ectodermal  plate,  the  auditory  placode,  located  midway  along  the  side  of 


20 


3o6 


THE  SENSE  ORGANS 


the  hind-brain  (Fig.  303  .4).  The  paired  placodes  are  invaginated  to 
form  hollow  vesicles  which  close  at  about  the  stage  of  3 mm.,  but  remain 
in  temporary  union  with  the  ectoderm  (Fig.  305  B). 

The  otocyst,  or  auditory  vesicle,  when  closed  and  detached,  is  nearly 
spherical.  Approximately  at  the  point  where  it  joined  the  ectoderm,  a 
recess,  the  endolymph  duct,  is  formed  and  then  shifted  to  a mesial  position 
(Figs.  306  and  307  a).  The  endolymph  duct  corresponds  to  that  of 
selachian  fishes,  which  remains  permanently  open  to  the  exterior.  In 
man,  its  extremity  is  closed  and  dilated  into  the  endolymph  sac  (Fig.  307  /). 

In  an  embryo  of  7 mm., 
the  vesicle  has  elongated,  its 
narrower  ventral  process  con- 
stituting the  anlage  of  the 
cochlear  duct  (Figs.  306  and 
307  a).  The  wider,  dorsal 
portion  of  the  otocyst  is  the 
vestibular  anlage , which,  shows 
indications  dorsally  of  the 
developing  semicircular  ducts 
(Fig.  307  a).  These  are 

formed  in  1 1 mm.  embryos 
as  two  pouches — -the  anterior 
and  posterior  ducts  from  a 
single]  pouch  at  the  dorsal 
border  of  the  otocyst,  the 
lateral  duct  later  from  a 
horizontal  outpocketing  (Fig.  307  c).  Centrally,  the  walls  of  these  pouches 
flatten  and  fuse  into  epithelial  plates,  but  canals  are  left  peripherally, 
communicating  with  the  cavity  of  the  vestibule.  Soon,  the  solid,  central 
portions  of  the  epithelial  plates  are  resorbed,  leaving  the  semicircular 
ducts  as  in  Fig.  307  d,  c.  Dorsally,  a notch  separates  the  anterior  and 
posterior  ducts.  Of  these,  the  anterior  is  completed  before  the  posterior; 
the  lateral  duct  is  the  last  to  develop. 

In  a 20  mm.  embryo  ^Fig.  307  e),  the  three  semicircular  ducts  are 
present  and  the  cochlear  duct  has  begun  to  coil  like  a snail  shell.  It  will 
be  seen  that  the  anterior  and  posterior  ducts  have  a common  opening 
dorsally  into  the  vestibule,  while  their  opposite  ends,  and  the  cranial  end 
of  the  lateral  duct,  are  dilated  to  form  ampulla:.  In  each  ampulla  is 
located  an  end  organ,  the  crista  ampullaris,  which  will  be  referred  to  later. 
By  a constriction  of  its  wall  the  vestibule  is  differentiated  into  a dorsal 
portion,  the  iitriculus,  to  which  are  attached  the  semicircular  ducts,  and 
a ventral  portion,  the  sacculus,  connected  with  the  cochlear  duct  (Fig. 


Fig.  306. — Section  through  the  otic  vesicle  of  a 7 mm. 
human  embryo  (His). 


endoly 


vestib. 

pouch 


mph 


I at  groove 
vestib.  p.- 


coch. 

pouch 


a.6-6mni,  lateral. 


lat.gruove 
■•C.5C  lat. 

cochlea 
b 9 mm.  larerdl. 


c sc, post,. 


c/'  <:ijO 


absorpt  foci 


ab’sorp, 

focus 


c.iimm.  lateral 


fi.  2omm  ;ateral. 


. 30Tnm. 


lateral. 


r}.  13  mm  lateral 


Fig.  307. — Stages  in  the  development  of  the  internal  ear  (Streeter).  X 25.  The 
figures  show  lateral  views  of  models  of  the  left  membranous  labyrinth — a at  6.6  mm. ; 6 at  9 
mm.;  c at  1 1 mm.;  d at  13  mm.;  e at  20  mm.;  and  / at  30  mm.  The  colors,  yellow  and  red,  are 
used  to  indicate  respectively  the  cochlear  and  vestibular  divisions  of  the  acoustic  nerve  and 
its  ganglia,  absorp.  focus,  Area  of  wall  where  absorption  is  complete;  crus,  crus  commune; 
c.sc.lat.,  ductus  semicircularis  lateralis;  c. sc. post.,  ductus  semicircularis  posterior;  c. sc. sup., 
ductus  semicularis  superior  or  anterior;  cochlea,  ductus  cochlearis;  coch.  pouch,  cochlear  anlage; 
endolymph.,  endolymph  duct;  sacc.,  sacculus;  sac.  endol.,  endolymph  sac;  utric.,  utriculus. 


THE  EAR 


307 


307  e).  At  30  mm.,  the  adult  condition  is  nearly  attained  (Fig.  307  /). 
The  sacculus  and  utriculus  are  more  completely  separated,  the  semicir- 
cular ducts  are  relatively  longer,  their  ampullae  more  prominent,  and  the 
cochlear  duct  is  coiled  about  two  and  a half  turns.  In  the  adult,  the 
utriculus  and  sacculus  become  completely  separated  from  each  other,  but 
each  remains  attached  to  the  endolymph  duct  by  a slender  canal  that 
represents  the  prolongation  of  their  respective  walls.  Similarly,  the 
cochlear  duct  is  constricted  from  the  sacculus ; the  basal  end  of  the  former 
becomes  a blind  process,  and  a canal,  the  ductus  reiiniens,  alone  connects 
the  two. 

The  epithelium  of  the  membranous  labyrinth  is  composed  at  first  of  a 
single  layer  of  low  columnar  cells.  At  an  early  stage,  fibers  from  the 
acoustic  nerve  grow  between  the  epithelial  cells  in  certain  regions,  and  these 
become  modified  into  special  sense  organs.  Such  end  organs  are  the 
crista:  ampuUares  in  the  ampullce  of  the  semicircular  ducts,  the  macula 
acustica  in  the  utriculus  and  sacculus,  and  the  spiral  organ  (of  Corti)  in 
' the  cochlear  duct. 

i The  criste  and  maculae  are  static  organs,  or  sense  organs  for  maintaining  equilibrium. 

In  each  ampulla,  transverse  to  the  long  axis  of  the  duct,  the  epithelium  and  underlying 
tissue  form  a curved  ridge,  the  crista  (Fig.  309).  The  cells  of  the  epithelium  are  differen- 
tiated into  sense  cells,  with  bristle-like  hairs  at  their  ends,  and  supporting  cells.  Arboriz- 
ing about  the  bases  of  the  sensory  cells  are  fibers  from  the  vestibular  division  of  the  acous- 
tic nerve  (Fig.  307/).  The  maculae  resemble  the  cristae  in  their  development,  save  that 
larger  areas  of  the  epithelium  are  differentiated  into  cushion-like  end  organs.  Over  the 
maculae,  concretions  of  lime  salts  may  form  otoconia  which  remain  attached  to  the  sensory 
bristles. 

The  true  organ  of  hearing,  the  spiral  organ,  is  developed  in  the  basal  epithelium  of  the 
cochlear  duct,  basal  having  reference  here  to  the  base  of  the  cochlea.  The  development 
of  the  spiral  organ  has  been  studied  carefully  only  in  the  lower  mammals.  According  to 

I Prentiss  (1913),  in  pig  embryos  of  5 cm.  the  basal  epithelium  is  thickened,  the  cells 
. becoming  highly  columnar  and  the  nuclei  forming  several  layers.  In  later  stages,  7 to  9 cm., 

II  inner  and  outer  epithelial  thickenings  are  differentiated,  the  boundary  line  between  them 
1 being  the  future  spiral  tunnel  (Fig.  308  .4).  At  the  free  ends  of  the  cells  of  the  epithelial 
I swellings  there  is  formed  a cuticular  structure,  the  tectorial  membrane,  which  appears  first 

in  embryos  of  4 to  5 cm.  The  cells  of  the  inner  (a.xial)  thickening  giv'e  rise  to  the 
epithelium  of  the  spiral  limbus,  to  the  cells  lining  the  internal  spiral  sulcus,  and  to  the  sup- 
•'  porting  cells  and  inner  hair  cells  of  the  spiral  organ  (Fig.  308  B,  C).  The  outer  epithelial 
I thickening  forms  the  pillars  of  Corti,  the  outer  hair  cells,  and  supporting  cells  of  the  spiral 

I organ.  Differentiation  begins  in  the  basal  turn  of  the  cochlea  and  proceeds  toward  the 
|i  apex.  The  internal  spiral  sulcus  is  formed  by  the  degeneration  and  metamorphosis  of  the 
|i  cells  of  the  inner  epithelial  thickening  which  lie  between  the  labium  vestibulare  and 
1|  the  spiral  organ  (Fig.  308  B,  C).  These  cells  become  cuboidal  or  flat,  and  fine  the  spiral 

I I sulcus,  while  the  tectorial  membrane  loses  its  attachment  with  them. 

I From  what  is  known  of  the  development  of  the  spiral  organ  in  human  embryos,  it 
> I follows  the  same  lines  of  development  as  described  for  the  pig.  It  must  differentiate 
i relatively  late,  however,  for,  in  the  cochlear  duct  of  a newborn  child  figured  by  Krause,  the 


THE  SENSE  ORGANS 


30S 


Fig.  S08. — Stages  in  the  differentiation  of  the  spiral  organ  of  the  pig  (Prentiss).  X about 
130.  A,  8.5  cm.;  B,  20  cm.;  C,  30  cm.  (near  term),  ep.s.sp.,  Epithelium  of  spiral  sulcus;  h.c., 
hair  cells;  i.ep.c.,  inner  epithelial  thickening;  i.li.c.,  inner  hair  cells;  i.pil.,  inner  pillar  of  Corti; 
lab.  vest.,  labium  vestibulare;  limb,  sp.,  limbus  spiralis;  m.bas.,  basilar  membrane;  m.  tect.,  mem- 
brana  tectoria;  7ii.vest.,  vestibular  membrane;  n.coch.,  cochlear  division  of  acoustic  nerve; 
o.ep.c.,  outer  epithelial  thickening;  o.h.c.,  outer  hair  cells;  s.sp.,  sulcus  spiralis;  sc.tymp.,  scala 
tympani;  st.II,  stripe  of  Hensen;  t.sp.  spiral  tunnel. 


THE  EAR 


309 


spiral  sulcus  and  the  spiral  tunnel  are  not  yet  present.  The  development  of  the  acoustic 
nerve  and  the  distribution  of  its  vestibular  and  cochlear  divisions  are  described  on  p.  2S0 
and  illustrated  in  Figs.  285  and  307. 

The  mesenchyme  surrounding  the  membranous  labyrinth  is  differen- 
tiated into  a ffbrous  basement  membrane,  which  lies  next  the  Epithelium, 
and  into  cartilage  which  envelops  the  whole  labyrinth.  At  about  the  tenth 
week,  the  cartilage  bordering  the  labyrinth  then  begins  a secondary  reversal 
of  development  whereby  it  returns  first  to  precartilage  and  next  to  a syncy- 
tial reticulum  which  becomes  the  open  tissue  of  the  perilymph  spaces 
(Streeter,  1918)  (Fig.  309).  The  membranous  labyrinth  is  thus  suspended 
in  the  fluid  of  the  perilymph  space.  The  cochlear  duct  appears  triangular 


Suspensory  I iga m e 

■! 

Semicircular  duct^-J-'] 


" • ••’  ' membrane 


Reticulum  from  dedijfer- 
entiated  cartilage 


Cartilage 


■f^^Perilymph  space 


; ’fijf 

AV'.’.. 

m-1 


, ' Perichondrium 


. !* 

rV'  . 

^ '*•  •***^»V  * ' **  -**"•*•  ‘ 


Fig.  309. — Development  of  the  perilymph  space  about  a semicircular  duct  of  a four-months’ 
human  fetus  (after  Streeter).  X 85. 


in  section,  for  its  lateral  wall  remains  attached  to  the  peripheral  bony 
labyrinth,  wTile  its  inner  angle  is  adherent  to  the  modiolus.  Large  peri- 
lymph spaces  are  formed  above  and  below  the  cochlear  duct ; the  upper  is 
the  scala  vestibidi,  the  lower  the  scala  tympani.  The  thin  wall  separating 
the  cavity  of  the  cochlear  duct  from  that  of  the  scala  vestibuli  is  the 
vestibular  membrane  (of  Reissner)  (Fig.  308).  Beneath  the  basal  epithe- 
lium of  the  cochlear  duct,  a ffbrous  structure,  the  basilar  membrane,  is 
differentiated  by  the  mesenchyme.  The  bony  labyrinth  is  produced  by 
the  conversion  of  the  cartilage  capsule  into  bone.  The  modiolus  is  excep- 
tional, however,  in  that  it  develops  directly  from  mesenchyme  as  a mem- 
brane bone. 


310 


THE  SENSE  ORGANS 


Malleus 


Br.  arch.  I 
{Meckel's  cartilage) 


The  Middle  Ear. —The  middle  ear  cavity  is  differentiated  from  the 
first  pair  of  pharyngeal  pouches,  which  appear  in  embryos  of  3 mm.  (Fig. 
87).  The  entodermal  pouches  enlarge  rapidly,  ffatten  horizontally,  and 
are  in  temporary  contact  with  the  ectoderm  (Fig.  SS).  During  the  latter 
part  of  the  second  month,  the  proximal  wall  of  each  pouch  constricts  to 
form  the  auditory  tube.  This  canal  lengthens  and  its  lumen  becomes 
slit-like  during  the  fourth  month.  The  blind  end  of  the  pouch  enlarges 
into  the  tytn panic  cavity;  it  is  surrounded  by  loose  connective  tissue,  in 
which  the  auditory  ossicles  are  developed  and  for  a time  lie  embedded. 
Even  in  the  adult,  the  ossicles,  muscles,  and  chorda  tympani  nerve  retain 
a covering  of  mucous  epithelium  continuous  with  that  lining  the  tympanic 

cavity.  The  pneumatic  cells  of  the 
mastoid  wall  are  evaginations  formed 
at  the  close  of  fetal  life. 

The  auditory  ossicles  develop  from 
the  condensed  mesenchyme  of  the  first 
and  second  branchial  arches.  Of  these, 
the  malleus  and  incus  are  differentiated 
serially  from  the  dorsal  end  of  the  first 
arch  (Figs.  233  and  310).  The  car- 
tilaginous anlage  of  the  malleus  becomes 
disconnected  from  Aleckel’s  cartilage  of  the  mandible  when  ossification 
begins.  A portion  of  the  incus,  which  in  early  stages  joins  the  stapes, 
becomes  the  crus  longum.  Articulations  develop  where  the  three  ossicles 
touch. 

The  stapes  is  derived  from  the  second  branchial  arch  (Fig.  310).  Its 
mesenchymal  and  cartilaginous  anlages  are  perforated  by  the  stapedial 
artery,  and  consequently  become  ring-shaped.  This  form  persists  until 
the  middle  of  the  third  month,  when  the  adult  structure  is  assumed  and 
the  stapedial  artery  disappears. 


ympanunr  ^ _ 

..Stapes 

Br.  arch  II  ^ 

(Reichert's  cartilage) 

Fig.  310. — Diagram  showing  the  branchial 
arch  origin  of  the  auditory  ossicles. 


The  muscle  of  the  malleus,  the  tensor  tympani,  is  derived  from  the  first  branchial  arch; 
the  stapedial  muscle  from  the  second  arch.  The  further  fact  that  these  muscles  are  inner- 
vated by  the  trigeminal  and  facial  nerves,  which  are  the  nerves  of  the  first  and  second 
arches  respectively,  points  toward  a similar  origin  for  the  ear  ossicles.  These  relations 
strengthen  the  belief  in  a branchial  arch  origin,  as  maintained  by  most  modern  investiga- 
tors. Fuchs  (1905)  is  among  those  who  deny  that  the  ossicles  are  derived  from  the  arches. 


The  External  Ear. — -The  external  ear  is  developed  from  the  first 
ectodermal  branchial  groove  and  its  adjoining  arches  (Fig.  64).  The 
external  acoustic  meatus  represents  the  groove  itself,  which,  for  a time,  is  in 
contact  with  the  entoderm  of  the  first  pharyngeal  pouch.  Later,  however, 
this  contact  is  lost,  and,  toward  the  end  of  the  second  month,  the  groove 
deepens  centralh*  to  form  a funnel-shaped  canal  which  corresponds  to  the 


THE  EAR 


3II 


outer  portion  of  the  definitive  meatus  (Fig.  65).  From  the  inner,  ectoder- 
mal surface  a cellular  plate  grows  back  and  reaches  the  tympanic  cavity. 
During  the  seventh  month  the  plate  splits,  and  the  space  thus  added  con- 
stitutes the  inner  portion  of  the  external  meatus. 

The  tympanic  membrane  forms  by  a thinning  out  of  the  mesodermal 
tissue  in  the  region  where  the  wall  of  the  external  auditory  meatus  abuts 
upon  the  wall  of  the  tympanic  cavity.  Hence,  it  is  covered  externally  b}^ 
ectodermal  epithelium  and  internally  by  entoderm. 

The  auricle  arises  from  six  elevations,  which  appear,  three  on  the 
mandibular  arch,  and  three  on  the  hyoid  arch  (Fig.  31 1).  Modern 


Fig.  31 1. — Stages  in  the  development  of  the  auricle  (adapted  in  part  after  His.)  .4, 
II  mm.;  B,  13.6  mm.;  C,  15  mm.;  D,  adult,  i,  2,  3,  Elevations  on  the  mandibular  arch;  4,  5,  6, 
elevations  on  the  hyoid  arch;  af,  auricular  fold;  ov,  otic  vesicle;  i,  tragus;  2,  3,  helix;  4,  5,  anti- 
helix; 6,  antitragus. 

accounts  of  the  transformation  of  these  hillocks  into  the  adult  auricle 
agree  in  the  main; 

Caudal  to  the  hyoid  anlages,  a fold  of  the  hyoid  integument  is 
formed,  the  auricular  fold,  or  hyoid  helix.  A similar  fold,  dorsal  to  the 
first  branchial  groove,  appears  later,  and  unites  with  the  auricular  fold 
to  form  with  it  the  free  margin  of  the  auricle.  The  point  of  fusion  of 
these  two  folds  marks  the  position  of  the  satyr  tubercle,  according  to 


312 


THE  SENSE  ORGANS 


Schwalbe.  Darwin's  tubercle  occurs  at  about  the  middle  of  the  margin  of  1 
the  free  auricular  fold,  and  corresponds  to  the  apex  of  the  auricle  in  lower  1 
mammals.  The  tragus  is  derived  from  mandibular  hillock  i ; the  helix  from  S 
mandibular  hillocks  2 and  3 ; the  antihelix  from  hyoid  hillocks  4 and  5 ; t 
the  antitragus  from  hyoid  hillock  6.  The  lobule  represents  the  lower  end  y. 
of  the  auricular  fold.  X 


PART  III.  A LABORATORY  MANUAL  OF 
EMBRYOLOGY^ 

CHAPTER  XVI 

THE  STUDY  OF  CHICK  EMBRYOS 
(A)  THE  UNINCUBATED  OVUM  AND  EMBRYOS  OF  THE  FIRST  DAY 

The  Unincubated  Egg.— The  ‘yolk’  of  the  hen’s  egg  is  a single  ovum, 
enormously  expanded  with  stored  food  material.  AVhen  this  egg  cell  is 
expelled  from  the  ovary,  at  the  time  of  ovulation,  it  is  enveloped  by  the 
vitelline  membrane,  secreted  by  the  cytoplasm,  and  by  the  delicated  zona 
pellucida,  a product  of  the  follicle  cells  (Fig.  312).  By  the  time  the 
liberated  ovum  passes  into  the  oviduct,  the  process  of  maturation  has 

Yellow  yolk  White  yolk  Blastoderm 


Inner  shell  membrane 

Air  chamber 

Shell 

Outer  shell  membrane 
Chalaza 


Vtileline  membrane  Abmluen 

Fig.  312. — Diagrammatic  longitudinal  section  of  a hen’s  egg  before  incubation  (Thomson  in 

Heisler). 


progressed  to  the  point  where  one  polar  cell  is  given  off  (cf.  Fig.  15  A). 
Fertilization  by  mature,  waiting  spermatozoa  now  ensues,  and,  coinci- 
dently,  the  second  polar  cell  is  extruded  to  complete  maturation  (cf. 
Figs.  15  B and  17).  As  the  egg  passes  down  the  oviduct,  the  albumen, 
shell  membrane,  and  shell  are  added  as  accessory  investments.  The  ovum 
is  ready  to  be  laid  one  day  after  its  discharge  from  the  ovary;  at  this  time, 
the  appearance  is  as  indicated  in  Fig.  312.  The  cytoplasmic  area,  already 

^ A majority  of  the  illustrations  for  this  section  and  the  skeleton  of  many  descriptions  have  been  adapted 
from  the  manual  published  by  Professor  C.  W.  Prentiss. 

313 


314 


THE  STUDY  OF  CHICK  EMBRYOS 


started  toward  the  formation  of  an  embryo,  is  the  familiar  whitish  disc, 
technically  designated  the  blastoderm. 


Blaslomere  Blastocale 


Fig.  313. — Cleavage  of  the  [ngeon’s  ovum  (redrawn  from  Blount).  A,  Blastoderm  in  surface 

view;  B,  in  vertical  section. 


Invaginated 

Yolk  Ectoder?)!  Blaslocwle  Archenteron  entoderm  Blasiopor 


Cephalad 


•V-^anHilxoui^ 

FiCi.  314. — Gastrulation  in  the  pigeon,  as  shown  by  a longitudinal  section  of  the  blastoderm 

(redrawn  after  Patterson).  X 50. 


Cleavage,  Blastula  and  Gastrula. — Fertilization  promptly  initiates  a 
series  of  cell  divisions  which  divide  the  blastoderm  into  a cellular  disc, 

separated  from  the  yolk  by  a cleft- 
like space  (Fig.  313).  Such  mitoses 
comprise  the  process  of  cleavage,  and 
the  resulting,  asymmetrical,  hollow 
sphere  is  the  blastula.  During  the 
period  of  gastrulation  which  follows, 
the  blastoderm  becomes  two-layered. 
This  is  accomplished  by  the  rolling 
under  of  cells  at  the  future  caudal 
margin  of  the  blastoderm  (Fig.  314). 
Such  proliferation  and  undertucking 
gives  rise  to  an  inner  layer,  the 
entoderm;  the  original  surface  layer  is 
the  ectoderm. 

Primitive  Streak  and  Mesoderm. 

The  first  conspicuous  structure  on 
the  blastoderm  is  an  opaque  band  which  is  named  the  primitive  streak 
(Fig.  315).  It  appears  after  16  hours  incubation,  lying  somewhat  caudad 


— Area  opaca 

Primitive  knot 
Primitive  pit 

1 Primitive  fold 

j Primitive  groove 

Area  pcllucida 
1 Blood  island 


Fig.  315. — Chick  blastoderm  at  the 
stage  of  the  primitive  streak  (16  hours). 
X 20. 


THE  UNINCUBATED  OVUM  AND  EMBRYOS  OF  THE  FIRST  DAY  315 

in  the  future  midline.  The  primitive  streak  is  interpreted  as  the  margin 
of  the  germinal  disc  where  entoderm  formation  just  occurred;  the  changed 
appearance  and  direction  are  due  to  the  swinging  together  and  fusion  of 
its  two  lateral  halves  (Fig.  30).  Directly  following  the  earliest  appear- 
ance of  the  streak,  a primitive  groove  courses  lengthwise  along  its  surface. 
Cephalad,  this  ends  in  the  deeper  primitive  pit,  while  at  the  extreme 
cranial  end  is  an  area  not  indented  by  the  groove,  known  as  the  primi- 
tive knot  (of  Hensen). 

Transverse  sections  across  the  primitive  streak  prove  that  it  is  a 
thickening  of  the  ectoderm  and  the  site  of  origin  for  the  middle  germ 
layer — -as  it  was  for  the  entoderm  at  an  earlier  stage  (Fig.  316).  When 
the  mesoderm  cells  first  arise,  they  are  sparse,  migratory  elements  which 
soon  associate  into  distinct  plates  extending  laterad  and  caudad.  Later, 
the  mesoderm  invades  the  region  in  front  of  the  streak.  At  the  primitive 
knot  all  three  germ  layers  fuse  intimately  (Fig.  316  A)\  in  the  caudal  half 


Ectoderm  Primitive  knot 


Fig.  316. — Transverse  sections  of  a chick  embryo  at  the  stage  of  the  primitive  streak.  X 165. 

.4,  Through  the  primitive  knot;  B,  through  the  primitive  groove. 

of  the  streak  the  entoderm  is  free  (Fig.  316  B).  But  at  both  levels  the 
mesoderm  represents  lateral  proliferations  from  the  primitive  streak.  It 
appears  that  the  primitive  groove  is  the  mechanical  result  of  this  rapid 
growth,  or  invagination,  of  mesoderm.  From  the  three  germ  layers, 
thus  formed,  all  the  tissues  and  organs  will  develop,  as  listed  on  p.  6. 

Head  Process  and  Head  Fold. — Embryos  of  about  19  hours’  incuba- 
tion show  an  axial  strand  of  cells  extending  forward  from  the  primitive 
knot  (Fig.  317).  This  is  the  so-called  head  process;  it  is  also  termed  the 
notochordal  plate  because  it  becomes  the  cylindrical  notochord  which 
serves  as  the  primitive  axis  about  which  the  embryo  differentiates.  Al- 
though the  head  process  is  often  described  as  an  outgrowth  from  the  primi- 


3i6 


THE  STUDY  OF  CHICK  EMBRYOS 


tive  knot,  it  more  probably  represents  a later  stage  of  the  cephalic  end  of 
the  primitive  streak,  and  grows  progressively  at  the  expense  of  the  primi- 
tive streak,  as  the  latter,  still  maintaining  its  regional  characteristics, 
recedes  caudad.  A longitudinal  section  shows  the  relation  of  head  process 
to  primitive  knot  (P^ig.  318);  a transverse  section  demonstrates  it  as  a 
median,  thicker  mass,  continuous  laterally  with  mesoderm  which  has 
grown  into  this  region  (Fig.  319).  Both  sections  illustrate  the  indepen- 


Fig.  317. — Chick  blastoderm  and  embryo  at  the  stage  of  the  head  process  and  head  fold 

( 19  hours).  X 19. 

dence  of  the  head  process  from  the  ectoderm  above,  and  the  temporary 
fusion  with  the  entoderm  below. 

After  the  head  process  is  established,  a curved  fold  appears  cephalad 
to  it  (Fig.  317).  This  is  the  head  fold  which  at  first  involves  ectoderm  and 
entoderm  alone  (Fig.  318).  The  future  development  of  this  important 


Fig.  318. — Median  sagittal  section  of  a chick  embryo  at  the  stage  of  the  head  process  and 

head  fold  (19  hours).  X 100. 


structure  will  establish  the  gut  internally  and  definitely  delimit  the  upper 
body  externally. 

Neural  Groove  and  Mesodermal  Segments. — Even  embryos  of  the 
previous  stage  exhibit  a broad  zone  of  thickening  in  the  ectoderm  over- 


THE  UNIN'CUBATED  OVUM  AND  EMBRYOS  OF  THE  FIRST  DAY  317 

lying  the  head  process.  This  region  constitutes  the  neural  plate  (Fig. 
319).  In  an  embryo  of  21  hours,  the  plate  folds  lengthwise  to  form  a 
gutter-like  trough,  called  the  neural  groove,  which  shortly  will  become 
rolled  into  the  tubular  brain  and  spinal  cord  (Fig.  320).  The  notochord 

Ectoderm  Neural  plate 


Mesoderm  Notochordal  plate  Entoderm 

Fig.  319. — Transverse  section  through  the  head  process  of  a 19  hour  chick  embryo.  X 165. 


now  shows  through  the  ectoderm  at  the  bottom  of  the  groove;  laterally, 
the  groove  is  flanked  by  elevated  ridges,  the  neural  folds. 

The  wings  of  mesoderm  which  grew  from  the  sides  of  the  primitive 
streak  have  spread  cephalad  to  the  extent  indicated  by  the  darker  shading 


Neural 

groove 


Primitive 

segment 

Primitive 

knot 


Primitive 

streak 


Fig.  320. — Chick  embryo  at  the  stage  of  the  neural  groove  and  first  two  pairs  of  segments 
(21  hours)  (after  Duval).  X 13.  The  head  process  is  visible  through  the  bottom  of  the 
neural  groove. 


in  Fig.  32c.  Next  the  notochord,  the  mesoderm  is  thick,  and  in  it  have 
appeared  two  pairs  of  vertical  clefts;- these  separate  the  mesoderm  into 
successive  masses  (the  first  incomplete  cranially),  which  will  be  seen 
better  in  older  stages.  The}^  are  mesodermal  segments. 


THE  STUDY  OF  CHICK  EMBRYOS 


318 


(B)  EMBRYO  OF  FIVE  SEGMENTS  (TWENTY -THREE  HOURS) 

It  is  evident  that  an  embryonic  and  an  extra-embryonic  region  of  the 
blastoderm  are  becoming  more  sharply  defined  (Fig.  321).  Of  the  extra- 
embryonic  territory,  that  nearest  the  embryo  comprises  the  clearer  area 
pelliicida;  peripherad  lies  the  area  opaca,  darker  because  of  its  adherence 
to  the  yolk  beneath.  In  the  more  proximal  zone  of  the  opaque  area  are 
mottled  masses,  the  blood  isla>ids,  already  observed  in  younger  stages 
but  now  fusing  into  an  incomplete  network.  This  mesh  is  best  developed 
caudally;  it  is  the  area  vascidosa. 

At  this  period  the  head  is  growing  rapidly.  It  rises  above  the  blasto- 
derm and  projects  ccphalad  as  a somewhat  cylindrical  part  of  the  embryo, 


Head 


Fig.  321. — Dorsal  view  of  a chick  embryo  with  hve  segments  (23  hours).  X 14-  1 

which,  at  its  cephalic  end,  is  entirely  free  (Fig.  321).  In  accomplishing 
this  result,  the  shallow  head  fold  of  earlier  stages  appears  to  have  grown  j 
caudad  and  liberated  the  head  by  undercutting  (Fig.  322) ; the  real  factor, 
however,  is  a true  forward  growth  on  the  part  of  the  head  itself.  Simul-  ;i 
taneously  with  the  extension  of  the  head,  the  entodermal  component  of 
the  original  head  fold  is  elongated  into  an  internal  tubular  pocket  of  | 
roughly  corresponding  shape;  this  is  the  primitive  j ore-gut.  Cranially,  j 
it  is  a blind  sac;  caudally,  it  opens  out  onto  the  yolk  through  an  arched  1 
aperture  termed  the  intestinal  portal.  In  Fig.  321  the  lateral  limits  of 


EMBRYO  OF  SEVEN  SEGMENTS  (XWENTY-FIVE  HOURS)  319 

the  darker  fore-gut  and  its  relation  to  the  crescentic  intestinal  portal  are 
shown  plainly;  Fig.  322  illustrates  how  the  entoderm  is  reflected  into  the 
fore-gut  at  the  level  of  the  portal. 

The  neural  groove  is  broad  and  deep  (Fig.  321).  Midway,  its  lateral 
folds  are  approximated  and  ready  to  fuse.  Caudally,  the  folds  diverge 
and  become  increasingly  indistinct. 

The  mesodermal  segments  are  clearly  defined  and  block-like.  The 
notochord  shows  through  the  transparent  ectoderm,  and  the  x^rimitive 
streak  is  shorter,  both  relatively  and  actually.  Later,  when  the  body 
form  is  further  indicated  by  the  formation  of  the  tail  fold,  the  primitive 
streak  will  disappear.  It  is  a notable  fact  that  the  head  not  only  arises 


Fore-^ut  Intest.  Nolo.  Mesen-  Prim.  Prim. 


Fig.  322. — Median  longitudinal  section  of  a five-segment  chick  embryo  (redrawn  from 

Patten).  X 25. 


soonest  but  retains  its  early  advantage  over  lower  levels  of  the  body. 
The  progressive  advance  of  differentiation  first  reaches  the  end  of  the 
trunk  at  a considerably  later  period. 

(Cj  EMBRYO  OF  SEVEN  SEGMENTS  (TWENTY -FIVE  HOURS) 

A surface  view  of  a chick  embryo  at  this  stage  resembles  the  one  last 
described,  but  shows  certain  distinct  advances  (Fig.  323) ; yet  the  descrip- 
tions that  follow  will  apply  in  all  essentials  to  embryos  having  from  five 
to  ten  primitive  segments.  The  vascular  area  of  the  blastoderm  is  better 
organized  than  before  and  extends  far  cephalad.  In  front  of  the  head  is 
a light  area,  not  yet  invaded  by  mesoderm  and  known  by  the  unsuitable 
name  proamnion.  The  primitive  streak  is  still  prominent  caudally  and 
measures  about  one-fourth  the  length  of  the  embryo.  The  notochord  may 
be  followed  cephalad  from  the  primitive  knot  until  it  is  lost  beneath  the 
neural  tube. 

Neural  Tube. — The  lips  of  the  neural  folds  have  met  throughout  the 
cranial  two-thirds  of  the  embryo,  but  have  not  fused  to  any  extent.  The 
neural  tube,  formed  thus  by  the  closing  of  the  ectodermal  folds,  is  open  at 
each  end;  the  delayed  closure  of  the  cranial  extremity  leaves  a temporary 
opening  to  the  outside,  designated  the  anterior  neiiropore.  In  succeeding 
stages,  the  more  caudal  regions  of  the  present  neural  groove  will  be  rolled 
progressively  into  a tube  and  added  to  that  already  completed.  At  the 


320 


THE  STUDY  OF  CHICK  EMBRYOS 


head  end,  the  neural  tube  has  begun  to  expand  into  the  brain  vesicles. 
Of  these,  only  ihe  forc-hrain  is  prominent,  and  from  it  the  optic  vesicles  are 
budding  out  laterally. 

Fore -gut.  lixcept  for  an  increase  in  size,  the  fore-gut  is  little  changed. 
Near  its  lilind  end,  the  floor  of  the  gut  is  applied  to  the  ectoderm  and  the 
two  comprise  the  temporary  pharyngeal  membrane  (cf.  Fig.  335).  The 


Anterior  nciiropore  Fore-brant 


Free  portion  of  head 


Neural  groove. 

325 


326 

Primitive  knot 


Blood  island 


fore-gut  will  ultimately  constitute  the  alimentary  canal  as  far  as  the  middle 
of  the  small  intestine,  ddie  way  in  which  the  entoderm  is  folded  up  from 
the  blastoderm  and  forward  into  the  head  is  shown  well  in  Figs.  322  and-- 
335- 

M esoderm  and  Coelom. — The  tissue  of  the  middle  germ  layer  assumes 
two  different  forms.  Throughout  most  of  the  head  region  it  comprises 
a diffuse  meshwork  of  cells  which  fills  in  the  spaces  between  the  various 
epithelial  layers.  This  tissue  is  mesenchyme  (Fig.  331).  In  the  caudal 


M csodcrmal 
segment  j 


Area  peltucida 

326 

Notochord 


Area  opaca 


Primitive  streak 

Fig.  323.  — Dorsal  view  of  a chick  embryo  with  seven  segments  (25  hours).  X 20. 
numbered  lines  indicate  the  levels  of  the  sections.  Figs.  325-332. 


Pharynx 


323  

Left  vitelline  vein 
33» 

329 

328  - 

327 


vitelline 


EMBRYO  OF  SEVEN  SEGMENTS  (xWENT Y-FIVE  HOURS) 


321 


part  of  the  head,  and  in  the  remainder  of  the  body,  the  mesoderm  at 
this  stage  is  organizing  more  definitely.  Nearest  the  midplane,  it  is  divided 
by  transverse  furrows  into  seven  block-like  primitive  segments,  four  of 
which  belong  to  the  future  head  (Figs.  323  and  324).  Caudad,  between  the 
segments  and  the  primitive  streak,  there  is  the  undifferentiated  mesoderm 
of  the  segmental  zone,  but  new  pairs  of  segments  will  develop  progressively 
throughout  this  region.  Lateral  to  each  segment  is  a plate  of  unsegmented 
mesoderm,  termed  the  intermediate  cell  mass;  it  is  also  called  the  neplirotome 
because  it  will  play  an  important  role  in  the  development  of  the  excretory 
system  (Fig.  324).  The  nephrotome  plate  serves  as  a bridge  between  the 
segments  and  the  unsegmented  lateral  mesoderm.  The  lateral  mesoderm, 


Coelom 

Fig.  324. — Diagrammatic  transverse  section  through  a hen’s  egg  at  an  early  stage  of  develop- 
ment (Minot-Prentiss). 

when  first  formed,  aggregates  into  two  solid  plates  (Fig.  316)  each  of 
which  splits  secondarily  into  two  lamellce,  separated  by  a space  (Figs. 
324  and  325).  The  dorsal  layer  comprises  the  somatic  mesoderm,  the 
ventral  layer  the  splanchnic  mesoderm. 

The  space  between  the  two  layers  first  occurs  as  isolated  clefts, 
which  soon  unite  to  form  the  body  cavity,  or  coelom  (Fig.  324).  The 
originally  bilateral  coelomic  chambers  will  later  become  confluent  v^en- 
trally,  as  in  the  adult  (Fig.  324).  In  the  region  of  the  heart,  the  ccelom  is 
already  enlarged  locally,  anticipating  its  destiny  as  the  pericardial  cavity. 
Other  portions  will  become  the  pleural  cavities  of  the  thorax,  and  the 
peritoneal  cavity  of  the  abdomen. 

Heart  and  Blood  Vessels. — The  heart  is  a simple,  straight  tube,  l3ung 
in  the  midplane,  ventral  to  the  gut.  In  a dorsal  view  of  the  total  embr^m 
it  is  inconspicuous  because  largely  concealed  by  overlying  structures. 


Notochord 


Neurol  fuhc 


Somato- 

plenre 


THE  STUDY  OF  CHICK  EMBRYOS 


322 

Caudally,  it  is  continuous  with  the  converging  vitelline  veins  which  enter 
the  body  by  following  along  the  margins  of  the  intestinal  portal:  the  two 
veins  unite  as  they  join  the  heart  (Fig.  323).  From  the  cephalic  end  of 
the  heart  is  given  off  the  ventral  aorta;  dorsal  to  the  gut  course  paired 
descending  aortcc. 

Transverse  Sections 

The  first  embryo  to  be  studied  in  serial  section  is  easiest  understood 
if  the  student  begins  caudad,  where  differentiation  is  least,  and  works 
toward  the  head.  Important  facts  pertaining  to  the  germ  layers  and  the 
jirinciples  underlying  the  development  of  the  neural  tube,  gut,  heart,  and 
head  are  then  made  simple.  The  following  illustrations  and  descriptions 
may  be  used  to  interpret  sections  of  chick  embryos  between  the  stages 
of  five  and  ten  segments.  The  level  of  each  section  may  be  determined 
from  the  numbered  lines  on  Fig.  323. 

Sections  through  the  Primitive  Streak  and  Knot.  —Conditions  are  essentially  the  same  , 
as  in  the  younger  embryos  already  examined  (Fig.  316).  ! 

Section  through  the  Fifth  Primitive  Segment  (Fig.  325). — This  general  level  is  charac- 
terized l.)y  the  differentiation  of  the  mesoderm,  the  approximation  of  the  neural  folds,  and  ! 


Neural  fold  Neural  groove 


Fig.  325. — Transverse  section  through  the  fifth  pair  of  mesodermal  segments  of  a seven- 

segment  chick  embryo.  X 90. 

the  presence  of  two  vessels,  the  descending  aortir,  one  on  each  side  between  the  mesodermal 
segments  and  the  entoderm.  The  neural  folds  are  thick,  as  is  the  adjoining  ectoderm  to  ii 

a less  degree.  The  jwtochord  is  a sharply  defined  oval  mass  of  cells.  The  mesodermal 
segments  are  somewhat  triangular  in  outline  and  connected  by  the  intermediate  cell  mass,  or  j 
iicplirotomc,  with,  the  lateral  mesoderm.  The  lateral  mesoderm  is  partially  divided  by  irregu-  j 
lar,  flattened  spaces  into  two  sheets,  the  dorsal  of  which  is  the  somatic  layer,  the  ventral 
the  splanchnic  layer.  Later,  the  spaces  unite  to  form  the  cwlom,  or  primitive  body  cavity,  J 
and  the  mesodermal  lining  then  becomes  mcsothelium.  || 

Through  the  higher  segments  in  the  series  the  differentiation  of  mesoderm  and  ccelom 
is  more  advanced  (cf.  Fig.  343).  Caudal  to  the  seventh  segment,  in  the  region  of  the 
segmental  zone,  the  mesoderm  forms  solid  plates  (cf.  Fig.  344). 

Sections  through  the  Area  Vasculosa  (Fig.  326). — The  illustrations  show  a little  of  the 
extra-embryonic  territory,  peripheral  to  the  area  pellucida.  In  this  region  of  the  area 
0 paca , the  entoderm  is  associated  intimately  with  the  coarsely  granular  yolk.  The  splanch- 


EMBRYO  OF  SEVEN  SEGMENTS  (tWENTY-FIVE  HOURS) 


323 


nic  mesoderm  contains  aggregations  of  cells  known  as  Mood  islands,  many  of  which  are 
fusing  into  the  network  that  characterizes  the  area  vasciilosa  (Figs.  323  and  326  .I).  The 
cellular  thickenings  of  the  blood  islands  undergo  differentiation  into  two  cell  types : fluid- 
filled  vacuoles  appear  and  expand  so  as  to  set  free  the  innermost  cells  which  later  separate 
and  float  about  as  primitive  Mood  corpuscles;  the  same  process  flattens  the  peripheral  cells 
into  an  endothelium  {B,  C).  The  endothelial  spaces  both  coalesce  and  form  new  vascular 
sprouts,  and  in  this  way  the  system  of  extra-embryonic  vessels  is  extended.  All  blood 
vessels  at  first  consist  of  an  endothelial  layer  only. 


Blood  island  Ectoderm  Somatic  mesoderm  Splanchnic  mesoderm  Blood  vessel  Blood  cells 


Fig.  326.  —Transverse  sections  through  the  area  vasculosa  of  a seven-segment  chick  embryo. 

X 300. 


Section  Caudal  to  the  Intestinal  Portal  (Fig.  327). — The  section  is  characterized:  (i) 
by  the  meeting  of  the  neural  folds  preparatory  to  closing  the  neural  tube;  (2)  by  the  arching 
of  the  entoderm,  which,  a few  sections  nearer  the  head  end,  forms  the  fore-gut;  (3)  by  the 
presence  of  the  vitelline  veins  laterally  between  the  entoderm  and  splanchnic  mesoderm; 


Fig.  327. — Transverse  section  caudal  to  the  intestinal  portal  of  a seven-segment  chick  embryo. 

X 90. 

(4)  by  the  wide  separation  of  the  somatic  and  splanchnic  mesoderm  and  the  consequent 
increase  in  the  size  of  the  coelom.  In  this  location  the  coelom  later  surrounds  the  heart  and 
is  converted  into  the  pericardial  cavity. 

The  neural  tube  at  this  level  is  transforming  into  the  third  brain  vesicle,  or  hind-brain. 
The  neural  folds  have  not  yet  fused,  and  at  their  dorsal  angles  are  the  neural  crests,  the 


324 


THE  STUDY  OF  CHICK  EMBRYOS 


anhiges  of  the  spinal  ganglia.  Mesodermal  segments  never  develop  as  far  cephalad  as 
this  region;  instead,  diffuse  masses  of  mesenchyme  occupy  comparable  positions  adjacent 
to  the  neural  tube.  On  the  left  of  the  section,  an  asterisk  marks  the  junction  of  somatic 
and  splanchnic  mesoderm. 

Section  through  the  Intestinal  Portal  (Fig.  328). — This  section  passes  through  a 
vertical  fold  of  entoderm  at  the  exact  point  where  the  latter  is  reflected  into  the  head  as  the 
forc-giit  (cf.  Figs.  322  and  335).  The  entoderm  forms  a continuous  bridge  of  tissue  between 


Neural  lube 


Descending  aorta 
Notochord 


Ectoderm 
Somatic  mesoderin 


Spl a n ch  11  ic  mesoderm 


C adorn 

Splanchnic  mesoderm^- 


dot  helium  of  vitelline  vein 


Entoderm 


Fig.  328. — Transverse  section  through  the  intestinal  portal  of  a seven-segment  chick  embryo. 

X 90. 


the  vitelline  veins,  thereby  closing  the  fore-gut  ventrally.  The  splanchnic  mesoderm  is 
differentiated  into  a thick-walled  pouch  on  each  side,  lateral  to  the  endothelial  layer  of  the 
veins. 

A few  sections  cephalad,  the  gut  separates  from  the  general  entoderm;  this  will  allow 
first  the  endothelial  heart  tubes  to  meet,  and  then  the  flanking  folds  of  splanchnic 
mesoderm. 


Ectoderm  Neural  tube 


Fig.  329. — Transverse  section  through  the  heart  of  a seven-segment  chick  embryo.  X 90. 

Section  through  the  Heart  (Fig.  320). — Passing  cephalad  in  the  series,  the  vitelline 
veins  open  into  the  heart  just  cranial  to  the  intestinal  portal.  The  entoderm  in  the  head 
fold  now  lines  the  crescentic  pharynx  of  the  fore-gut,  and  is  separated  by  the  heart,  ccelom, 
and  splanchnic  mesoderm  from  the  entoderm  of  the  germinal  disc.  The  descending 
aortae  are  larger,  making  conspicuous  spaces  between  the  neural  tube  {hind-brain)  and  the 
pharynx.  The  heart,  as  will  be  seen,  results  from  the  union  of  two  endothelial  tubes,  con- 


EMBRYO  OF  SEVEX  SEGMENTS  (TWENTY-FIVE  HOURS) 


325 


tinuous  with  those  constituting  the  vitelline  veins  in  the  preceding  sections.  The  median 
walls  of  these  tubes  disappear  at  a slightly  later  stage  and  thereby  establish  a single  tube, 
the  endocardium.  Thickened  layers  of  splanchnic  mesoderm,  which,  in  the  preceding 
section,  invested  the  vitelline  veins  laterally,  now  form  the  mesothelial  wall  of  the  heart; 
this  tissue  will  become  the  later  myocardium  and  epicardium.  In  the  median  ventral  plane, 
the  layers  of  splanchnic  mesoderm  of  each  side  have  fused  and  separated  from  the  splanch- 
nic mesoderm  of  the  germinal  disc;  thus,  the  two  pericardial  cavities  are  put  in  communi- 
cation. Dorsally,  the  splanchnic  mesoderm,  as  the  dorsal  mesocardiiim,  suspends  the  heart, 
while  still  more  dorsally  it  is  continuous  with  the  somatic  mesoderm  at  the  point  where 
the  mesenchyme  of  the  head  extends  to  the  coelom. 


Fig.  330. — Transverse  section  through  the  head  fold  of  a seven-segment  chick  embryo.  X 90. 


Origin  of  the  Heart  and  Embryonic  Vessels. — From  the  two  sections  last  described, 
it  is  seen  that  the  heart  arises  as  a pair  of  endothelial  tubes  lying  in  folds  of  the  splanchnic 
mesoderm.  Later,  the  endothelial  tubes  fuse  and  the  mesodermal  folds  are  also  brought 
together.  The  heart  then  consists  of  a single  endothelial  tube  within  a thick-walled 
investment  of  mesoderm.  The  endothelial  cells  of  the  heart  often  appear  to  arise  from 
the  entoderm  but  this  is  perhaps  a deception,  for  elsewhere  endothelium  is  mesodermal  in 


Ectoderm 


Entoderm  of 
proamnion 


Descending  aorta 


— — Fore-gut 


Ectoderm  of  proamnion 


Fig.  331. — Transverse  section  through  the  pharyngeal  membrane  of  a seven-segment  chick 

embryo.  X 90. 


origin.  The  vascular  system  is  primitively  a paired  system,  the  heart  arising  as  a double 
tube  with  two  veins  entering  and  two  arteries  leaving  it  (cf.  Figs.  180  and  181).  The 
blood  vessels  of  the  body  are  delicate  endothelial  channels  which  originate  as  clefts  in  the 
mesenchyme.  Coalescence  and  budding  produce  a plexus  from  which  definite  vessels  are 
selected  (Fig.  179). 

Section  through  the  Head  Fold  (Fig.  330). — It  will  be  remembered  that  a crescentic 
ectodermal  fold  lies  both  beneath  the  head  and  lateral  to  it,  and  that  the  portion  of  the  body 


326 


THE  STUDY  OF  CHICK  EMBRYOS 


cephalad  to  this  head  (old  is  free  from  the  blastoderm  (Figs.  322  and  323).  The  present 
section,  from  a level  just  cephalad  of  the  heart,  is  located  at  the  critical  region  where  these 
folds  meet.  The  inspection  of  a few  sections  in  each  direction  will  demonstrate  how  the 
embryonic  and  extra-embryonic  territories  are  related  and  how  they  become  separate. 
The  coelom  does  not  extend  into  the  head.  Midway  of  the  blastoderm  is  a space  which 
lacks  mesoderm;  it  is  the  proamnion.  Ventral  to  the  pharynx,  the  ventral  aorta  are  becom- 
ing separate  vessels  as  they  leave  the  heart. 

Section  through  the  Pharyngeal  Membrane  (Fig.  331). — This  section  shows  the  head 
free  from  the  underlying  blastoderm  (cf.  Fig.  322).  The  ectoderm  surrounds  the  head, 
and  near  the  midventral  line  it  is  bent  dorsad,  is  somewhat  thickened,  and  comes  in  con- 
tact with  the  thick  entoderm  of  the  pharynx.  The  area  of  contact  between  ectoderm  and 
pharyngeal  entoderm  constitutes  the  pharyngeal  membrane.  Later,  this  plate  breaks 


Fig.  332. — Transverse  section  through  the  fore-brain  and  optic  vesicles  of  a seven-segment 

chick  embryo.  X 90. 

through  and  establishes  the  oral  cavity,  which,  accordingly,  is  partly  ectodermal.  The 
expanded  neural  tube  is  closed  and  forms  the  middle  brain  vesicle,  or  mid-brain;  the  |i 
superficial  ectoderm  is  entirely  separate  from  it.  The  descending  aortse  appear  as  small 
vessels  dorsal  to  the  lateral  folds  of  the  pharynx.  The  blastoderm  in  the  region  beneath 
the  head  is  composed  of  ectoderm  and  entoderm  only;  this  is  the  proamnion.  Laterad  ' 

may  be  seen  the  layers  of  the  mesoderm.  i 

Section  through  the  Fore -brain  and  Optic  Vesicle  (Fig.  332). — The  neural  tube  is  open 
here  and  constitutes  the  first  brain  vesicle,  or  fore-brain.  The  opening  is  the  temporary 
anterior  neuroporc.  The  ectoderm  is  composed  of  two  or  three  layers  of  nuclei  and  is 
continuous  at  the  neuropore  with  the  much  thicker  wall  of  the  fore-brain.  The  two  ecto- 
dermal layers  are  in  contact  with  each  other  except  in  the  midventral  region,  where  the 
mesenchyme  is  beginning  to  penetrate  and  separate  them.  The  lateral  expansions  of  the  : 
fore-brain  are  the  optic  vesicles,  which  eventually  give  rise  to  the  retina  of  the  eye. 

(Dj  EMBRYO  OF  SEVENTEEN  SEGMENTS  (THIRTY-EIGHT  HOURS) 

The  stage  selected  as  a type  for  illustrating  the  significant  advances  j 
since  the  seven-segment  embryo  is  a chick  of  about  38  hours  incubation 
which  possesses  17  primitive  segments.  At  this  time,  the  segments  are  j| 
developing  rapidly  and  the  descriptions  that  follow  will  apply  satisfactorily  i 
to  embryos  between  33  hours  (12  segments)  and  40  hours  (18  segments). 

The  long  axis  of  the  embryo  is  still  nearly  straight,  but  specimens  of 
full  17  segments  should  show  a flexing  of  the  head  ventrad  (Fig.  335) 
and  a slight  turning  of  the  tip  of  the  head  on  its  left  side.  Fig.  333  does 
not  illustrate  this  feature.  The  area  pellucida  is  dumb-bell  shaped  and  is 
developing  a vascular  network.  The  extra-embryonic  vessels  of  the  area 


EMBRYO  OF  SEVENTEEN  SEGMENTS  (tHIRTY-EIGHT  HOURS) 


327 


opaca  are  well  differentiated,  and  the  vascular  area  is  bordered  by  a 
terminal  sinus.  Opposite  the  caudal  end  of  the  heart,  the  vascular  networks 
converge  and  become  continuous  with  the  stems  of  the  vitelline  veins. 
Connections  have  been  established  also  between  the  descending  aortae 
and  the  vascular  area  at  the  level  of  the  lowest  segments,  but  as  yet  the 
vitelline  arteries  have  not  appeared  as  distinct  trunks  (Fig.  334).  The 
tubular  heart  is  bent  to  the  embryo’s  right;  the  head  is  more  prominent 
and  the  three  primary  vesicles  of  the  brain  are  evident;  the  proamniotic 


Fore-brain 

Mid-brain 

Hind-brain 

Vitelline  vein 

Mesodermal 
segment  8 


Notochord 


Proamnion 
Optic  vesicle 
Free  portion  of  head 

Heart 


A enrol  tube 


Capillary  plexus  of 
future  vitelline’} 
artery 


Rhomboidal  sinus 


Primitive  streak 


Fig.  333. — Dorsal  view  of  a chick  embr\^o  with  seventeen  segments  (38  hours). 


X 20. 


area  is  reduced  to  a small  region  in  front  of  the  head ; the  primitive  streak 
is  inconspicuous. 

Central  Nervous  System  and  Sense  Organs. — ^The  tardy  closure  of 
the  anterior  neuropore  has  occurred,  and  the  neural  tube  is  complete  save 
at  its  caudal  end  where  the  divergent  neural  folds  form  the  so-called  rhom- 


328 


THE  STUDY  OF  CHICK  EMBRYOS 


boidal  sinus  (Fig.  333)-  In  the  head,  the  neural  tube  is  differentiated  into 
three  brain  vesicles,  marked  off  from  each  other  by  constrictions.  The 
fore-brain  (prosencephalon)  is  characterized  by  the  outgrowing  optic 
vesicles.  The  mid-brain  (mesencephalon)  is  a simple  dilatation.  The 
elongate  hind-brain  (rhombencephalon)  gradually  merges  with  the  spinal 
cord;  it  shows  a number  of  secondary  constrictions,  the  neiiromeres. 


ectoderm  of  the  ventral  surface  of  the  head  and  the  entoderm  caudal  to  the  intestinal  portal  have 
been  removed.  Numbered  lines  indicate  the  levels  of  Figs.  337—344. 

The  ectoderm  is  thickened  laterally  over  the  optic  vesicles  to  form 
the  lens  placode  of  the  eye  (Fig.  337).  The  optic  vesicle  flattens  at  this 
point  and  will  soon  invaginate  to  produce  the  optic  cup.  Dorso-laterally, 


EMBRYO  OF  SEVENTEEN  SEGMENTS  (tHIRTY-EIGHT  HOURS)  329 

in  the  hind-brain  region,  the  ectoderm  is  thickened  and  indented  as  the 
auditory  placodes  (Fig.  334).  Each  placode  will  become  an  otocyst,  or  otic 
vesicle,  from  which  differentiates  the  sensory  epithelium  of  the  internal  ear 
(membranous  labyrinth). 

Fore -gut. — The  entoderm  is  still  flattened  over  the  surface  of  the  yolk, 
caudal  to  the  intestinal  portal.  In  Fig  334,  the  greater  part  of  the  ento- 
derm is  cut  away.  The  broad  fore-gut,  folded  inward  at  the  portal,  shows 
indications  of  three  lateral  diverticula,  the  pharyngeal  pouches.  Cephalad, 
the  pharynx  is  closed  ventrally  by  the  pharyngeal  mcnihrane,  and  the 
ectodermal  depression  external  to  it  is  the  stoniodenm  (Fig.  335). 


Hind-hrain  Fcre-gut  Neural  tube 


Fig.  335. — Median  sagittal  section  of  the  head  end  of  a seventeen-segment  cliick  embryo. 

X about  50. 


Heart  and  Blood  Vessels. — After  receiving  the  vitelline  veins  just 
cephalad  of  the  intestinal  portal,  the  venous  end  of  the  heart  tube  dilates 
into  the  ventricle  which  bends  ventrad  and  to  the  embryo’s  right  (Fig. 
334).  It  then  is  flexed  dorsad  and  to  the  median  plane,  and  narrows  to 
form  the  bulbns,  and  its  continuation,  the  ventral  aorta.  The  ventral  aorta 
lies  beneath  the  pharynx  and  divides  into  two  divisions.  These  diverge 
and  course  dorsad  around  the  pharynx  as  the  first  pair  of  aortic  arches. 
Before  reaching  the  optic  vesicles  they  bend  sharply  caudad,  and,  as  the 
paired  descending  aorta:,  may  be  traced  to  a point  opposite  the  last  primitive 
segments.  In  the  region  of  the  intestinal  portal  they  lie  close  together 
and  have  fused  to  form  a single  vessel,  the  dorsal  aorta.  Below  this  level 
they  separate  again,  and,  opposite  the  last  primitive  segments,  connect 
by  numerous  capillaries  with  the  vascular  network.  In  this  region,  the 
trunks  of  the  paired  vitelline  arteries  presently  will  be  differentiated.  The 
heart  beats  spasmodically  at  this  stage;  the  blood  flows  from  the  vascular 
area  by  way  of  the  vitelline  veins  to  the  heart,  thence  by  the  aortse  and 
vitelline  arteries  back  again.  This  constitutes  the  vitelline  circulation, 
and  through  it  the  embryo  receives  nutriment  from  the  yolk  for  its  future 
development. 


330 


THE  STUDY  OF  CHICK  EMBRYOS 


Heretofore,  the  body  of  the  embryo  has  been  without  definite  veins, 
but  now  two  pairs  of  vessels  are  developing  for  the  purpose  of  returning 
lilood  to  the  heart.  The  anterior  cardinal  veins  drain  blood  from  the  head 
region;  the  posterior  cardinals,  just  appearing  at  this  stage,  will  perform  a 
similar  function  in  the  lower  body  (cf.  Fig.  348).  The  two  vessels  unite 
on  each  side  into  a common  cardinal  (duct  of  Cuvier)  which  enters  the 
venous  end  of  the  heart. 

Differentiation  of  Mesoderm.  -The  production  of  early  mesodermal 
segments,  and  the  addition  of  new  ones  by  a progressive  furrowing  of  the 
segmental  zone,  has  been  observed  in  previous  stages.  The  segments 
thus  formed  are  block-hke  with  rounded  corners  when  viewed  dorsally, 


Fig.  336. — Reconstruction  through  the  lower  mesodermal  segments  of  a two-day  chick  embryo. 

The  ectoderm  is  removed  from  the  dorsal  surface. 

triangular  in  transverse  section  (Fig.  336).  In  higher  vertebrates,  the 
segments  contain  at  most  only  indications  of  a cavity;  in  the  chick  there  is 
a minute  central  space,  representing  a portion  of  the  coelom,  which  is  filled 
with  a cellular  core,  while  the  other  cells  of  the  segment  form  a thick, 
radially-arranged  layer  about  it  (Fig.  343).  The  ventral  wall  and  a por- 
tion of  the  median  wall  of  each  primitive  segment  break  down  into  a 
mass  of  mesenchyme  termed  the  sclerotome  (Fig.  2 1 1 ) ; these  later  surround 
the  notochord  and  neural  tube,  and  transform  into  the  axial  skeleton.  The 
remaining  portions  of  the  segment  constitute  the  dernio-niyotome  (Figs. 
212  and  340).  The  cells  of  the  dorso-mesial  wall  of  the  plate,  the  myotome, 
eventually  give  rise  to  the  skeletal  musculature  of  the  body.  The  lateral 
plate  is  the  dermatome  which  is  destined  to  furnish  the  deeper  layers  of  the 
integument. 


EMBRYO  OF  SEVENTEEN  SEGMENTS  (tHIRTY-EIGHT  HOURS)  33 1 

The  bridge  of  cells  connecting  a primitive  segment  with  the  lateral 
mesodermal  layer  constitutes  the  intermediate  cell  mass,  or  nephrotome  (Fig. 
336).  In  the  chick,  the  nephrotomes  of  the  fifth  to  sixteenth  segments 
give  rise  to  segmental  pairs  of  bud-like  sprouts  which  extend  dorsad  (Fig. 
343).  These  are  the  prone phric  kidney  tubules.  Although  rudimentary, 
their  ends  unite  to  form  a tube,  known  as  the  pronephric  duct,  which  grows 
to  the  cloaca  (Fig.  128).  More  caudal  nephrotomes  will  soon  form  the 
embryonic  kidney,  or  mesonephros,  whose  tubules  open  into  the  pronephric 
duct,  then  called  the  mesonephric  duct  (Fig.  336).  Later  still,  the  perma- 
nent kidney  develops  partly  from  the  pronephric  duct  and  partly  from 
nephrotome  tissue.  Accordingly,  the  intermediate  cell  masses  may  be 
regarded  as  the  anlages  of  the  urogenital  glands  and  ducts — all  mesodermal 
in  origin. 

In  the  embryo  of  seven  primitive  segments,  the  lateral  mesoderm 
was  observed  to  split  into  two  layers,  the  dorsal  somatic  and  the  ventral 
splanchnic  mesoderm  (Fig.  336).  These  layers  persist,  the  somatic  meso- 
derm giving  rise  to  the  pericardium,  parietal  pleura,  and  peritoneum,  while 
the  splanchnic  layer  forms  the  epi-myocardium,  the  visceral  pleura,  and 
the  mesenteries  and  mesodermal  layers  of  the  gut.  The  somatic  meso- 
derm and  ectoderm  are  closely  associated  in  development  and  together 
are  designated  the  somatopleure  (Fig.  324) ; it  forms  the  body  wall.  Simi- 
larly, the  splanchnic  mesoderm  and  entoderm  are  jointly  termed  the 
splanchnoplciire.  Both  the  mesodermal  segments  and  the  unsegmented 
mesodermal  layers  contribute  the  mesenchymal  cells  which  play  such  an 
important  part  in  development. 

Transverse  Sections 

In  studying  serial  sections  of  an  embryo  it  is  not  sufficient  merely  to 
identify  the  structures  seen.  The  student  should  determine  also  the  exact 
level  of  each  significant  section  with  respect  to  the  illustrations  of  the  total 
embryo,  as  indicated  for  this  series  along  the  margins  of  Fig.  334,  and 
trace  the  organs  from  section  to  section  in  the  series.  He  is  then  ready  to 
reconstruct  mentally  the  complete  picture  of  a part  and  to  interpret  its 
origin  and  relations. 

The  following  sections  are  drawn,  viewed  from  the  cephalic  surface; 
hence,  the  right  side  of  the  embryo  is  at  the  reader’s  left.  These  illustra- 
tions and  descriptions  may  be  used  for  the  stud}"  of  chick  embryos  between 
33  hours  (12  segments)  and  40  hours  (18  segments). 

Section  through  the  Fore-brain  and  Optic  Vesicles  (Fig.  337). — The  optic  stalks 
connect  the  optic  vesicles  laterally  with  the  ventral  portion  of  the  fore-brain.  Dorsally. 
the  section  passes  through  the  mid-brain,  due  to  the  somewhat  ventrally  flexed  head  (cf. 
fiig-  335)-  The  lens  placodes  are  thickenings  of  the  surface  ectoderm  over  the  optic 
vesicles.  Note  that  there  is  now  a considerable  amount  of  mesenchyme  between  the 


332 


THE  STUDY  OF  ( HICK  EMBRYOS 


ectoderm  and  the  neural  tulje;  the  small  sj)aces  are  terminal  branches  of  the  anterior 
cardinal  veins.  Layers  of  mesoderm  are  present  in  the  underlying  blastoderm. 

Section  through  the  Pharyngeal  Membrane  and  Mid-brain  (Fig.  338). — In  the  mid- 
ventral  line,  the  thickened  ectoderm  bends  up  into  contact  with  the  entoderm  of  the 
rounded  pharynx  of  the  fore-gut.  The  resulting  ectodermal  pit  is  the  stomodeiiin,  and  the 
two  ajjposed  layers  represent  the  pharyngeal  membrane.  At  this  point  the  oral  opening  will 


Fig.  337. — Transverse  section  through  the  fore-brain  of  a seventeen-segment  chick  embryo. 

X 75 


Fig.  338. — Transverse  section  through  the  pharyngeal  memlirane  of  a seventeen-segment  chick 

embryo.  X 7,3. 

lireak  through.  ( )n  either  side  of  the  jiharynx  a pair  of  large  vessels  is  seen;  the  ventral 
pair  are  the  ventral  aorta.  Two  sections  cephalad,  their  cavities  open  into  those  of  the 
dorsal  pair,  the  descending  aorta.  The  section  is  thus  just  caudad  of  the  finst  aortic  arches. 
The  caudal  end  of  the  mesencephalon  is  the  portion  of  the  neural  tube  showing;  it  is  thick- 
walled,  with  an  oval  cavity.  Note  the  large  amount  of  undifferentiated  mesenchyme 


EMBRYO  OF  SEVENTEEN  SEGMENTS  (tHIRTY-EIGHT  HOURS) 


333 


throughout  the  section.  The  structure  of  the  blastoderm  is  complicated  by  the  presence 
of  collapsed  blood  vessels. 

Section  through  the  Hind-brain  and  Auditory  Placodes  (Fig.  339). — This  section  is 
characterized  by:  (i)  the  auditory  placodes,  which  represent  the  anlages  of  the  internal  ear; 


Fig.  339. — Transverse  section  through  the  hind-brain  and  auditory  placodes  of  a seventeen- 

segment  chick  embryo.  X 75. 

(2)  the  large  hind-hrain,  somewhat  thin  and  flattened  dorsad;  (3)  the  broad  pharynx,  cut 
through  the  second  pair  of  pharyngeal  pouches,  above  which  on  each  side  lie  the  descending 
aortcc;  (4)  the  presence  of  the  bulbar  and  ventricular  portions  of  the  heart.  The  bulbus  is 
suspended  dorsally  by  the  mesoderm,  which  here  forms  the  dorsal  mcsocardiuni.  The 


Ectoderm  Hind-hrain 


Fig.  340. — Transverse  section  through  the  caudal  end  of  the  heart  of  a seventeen-segment 

chick  embryo.  X 73. 


ventricle  lies  on  the  right  side  of  the  embryo;  a few  sections  caudad  in  the  series  it  is  con- 
tinuous with  the  bulbus  (cf.  Fig.  334).  Between  the  somatic  and  splanchnic  mesoderm  is 
the  large  pericardial  cavity,  surrounding  the  heart.  Yentro-lateral  to  the  brain  are  the 
anterior  cardinal  veins,  which  return  blood  from  the  head  region. 


THE  STUDY  OF  CHICK  EMBRYOS 


Section  through  the  Caudal  End  of  the  Heart  (Fig.  340).— The  section  still  includes 
the  hind-brain.  The  descending  aortce  are  separated  only  by  a thin  septum  which  is 
rujitured  at  this  level.  The  a)ilerior  cardinal  veins  are  cut  where  they  bend  ventrad  to 
enter  the  heart.  The  mesothelial  wall  of  the  heart  is  continuous  through  the  dorsal 
mesocardiiini  with  the  splanchnic  mesoderm.  On  the  right  side  of  the  section  there  is 
fusion  between  the  cpi-myocardiinn  of  the  heart  and  the  somatic  mesoderm.  iMesodermal 
segments  were  not  observed  at  higher  levels,  but  now  they  appear  lateral  to  the  hind-brain. 


Spinal  cord  Central  canal 


Fic..  342. — Tran.sverse  section  caudal  to  the  intestinal  portal  of  a seventeen-segment  chick 

embryo.  X 90. 

The  ventro-mesial  part  of  the  segment  is  breaking  down  into  the  sclerotome;  the  dorso- 
mesial  wall  represents  the  myotome,  and  the  lateral  plate  the  dermatome. 

Section  through  the  Intestinal  Portal  (Fig.  341). — The  descending  aortre  now  form  a 
single  vessel,  the  dorsal  aorta,  the  medium  septum  having  disappeared.  The  section  passes 
through  the  entoderm  at  the  point  where  it  is  folded  dorsad  and  cephalad  into  the  head  as 


EMBRYO  OF  SEVENTEEN  SEGMENTS  (THIRTY-EIGHT  HOURS) 


335 


the  fore-gut  (cf.  Fig.  335).  Two  sections  caudad  is  found  the  opening  {intestinal  portal) 
where  the  fore-gut  communicates  with  the  flattened  open  gut  between  the  entoderm  and 
the  yolk.  On  each  side  of  the  fore-gut  are  the  large  vitelline  veins,  sectioned  obliquely. 
The  splanchnic  mesoderm  overlying  these  veins  is  pressed  by  them  against  the  somatic 
mesoderm,  and  the  cavity  of  the  ccelom  is  thus  interrupted  on  each  side.  The  section  is 
close  to  the  level  where  the  common  cardinal  veins  open  into  the  venous  end  of  the  heart. 

Section  Caudal  to  the  Intestinal  Portal  (Fig.  342). — In  general,  this  section  resembles 
the  preceding  save  that  the  primitive  gut  is  without  a ventral  wall,  and,  therefore,  may  be 


Spinal  cord 


Mesodermal  segment 
Central  cells  of  segment 

Somatic  mesoderm 


Ectoderm 

Pronephric  tubule 


Splanchnic  mesoderm 

Descending  aorta'  I Entoderm 

Notochord 


Cceloyn. 


Fig.  343. — Transverse  section  through  the  fourteenth  pair  of  mesodermal  segments  of  a 
seventeen-segment  chick  embryo.  X 90. 


called  mid-gut.  The  right  vitelline  vein  is  still  large.  Lateral  to  the  enclosed  coelom,  on 
each  side,  are  spaces  which  represent  the  posterior  cardinal  veins,  just  differentiating. 

Section  through  the  Fourteenth  Pair  of  Primitive  Segments  (Fig.  343). — The  body  of 
the  embryo  is  now  flattened  on  the  surface  of  the  yolk.  Here  the  descending  aorta:  are 
again  separate  and  occupy  arched  spaces  under  the  primitive  segments.  The  section 
is  characterized  by  the  notochord  and  the  differentiated  mesoderm  which  forms  typical 
primitive  segments,  nephrotomes,  and  somatic  and  splanchnic  mesoderm.  Arising  from  the 


Fig.  344. — Transverse  section  through  the  segmental  zone  of  a seventeen-segment  chick 

embryo.  X 90. 

nephrotomes  are  sprout -like  pronephric  tubules.  The  tips  of  these  hollow  out  and  unite  to 
I form  the  primary  excretory,  or  pronephric  duct. 

i Section  through  the  Segmental  Zone  (Fig.  344). — The  section  is  at  the  level  of  the 

segmental  zone,  where  mesodermal  segments  have  not  formed  as  yet.  The  mesodermal 
' plates  are  splitting  laterally  into  layers,  but  the  ccelomic  cavities  are  mere  slits.  Between 
I the  splanchnic  mesoderm  and  the  entoderm,  blood  vessels  may  be  seen.  The  open  neural 
1 groove  is  called  the  rhomboidal  sinus.  The  ectoderm  is  characterized  by  the  columnar 


THE  STUDY  OF  CHICK  EMBRYOS 


336 

form  of  its  cells.  At  the  point  where  the  ectoderm  joins  the  neural  fold,  a ridge  of  cells 
projects  ventrally  on  either  side.  These  projecting  cells  constitute  the  neural  crests,  and 
from  them  the  spinal  ganglia  are  formed  in  older  embryos. 

Section  through  the  Primitive  (Hensen’s)  Knot  (Fig.  345). — The  three  germ  layers 
fuse  inseparably  at  the  'knot’  into  a mass  of  undifferentiated  tissue.  The  lateral 
mesoderm  is  split  into  somatic  and  splanchnic  layers;  the  latter  contains  numerous  small 
l)lood  vessels  of  the  vascular  network. 


Somatic  mesoderm  Ectoderm  Primitive  knot 


ha;.  345-  Transverse  section  through  the  primitive  knot  of  a seventeen-segment  chick 

embryo.  X 90. 

Section  through[the  Primitive  Streak  (Fig.  346). — In  the  mid-dorsal  line  is  the  primi- 
tive groove.  The  germ  layers  may  be  seen  taking  their  origin  from  the  undifferentiated 
tissue  of  the  primitive  streak,  beneath.  Laterad,  between  the  splanchnic  mesoderm  and 
entoderm,  lilood  ves.sels  are  present  as  in  the  preceding  sections. 


Somatic  mesoderm  Primitive  groove 


Fio.  346. — Transverse  section  through  the  primitive  streak  of  a seventeen-segment  chick^ 

emljryo.  X 90.  h' 

(E)  EMBRYO  OF  TWENTY -SEVEN  SEGMENTS  (TWO  DAYS) 

Although  a chick  embryo  with  27  segments  is  chosen  as  the  norm, 
the  descriptions  which  follow  are  applicable  to  stages  between  45  hours 
(23  segments)  and  60  hours  (32  segments). 

During  the  latter  half  of  the  second  day  a remarkable  change  occurs 
in  the  appearance  of  the  embryo  and  in  its  positional  relation  to  the 
blastoderm  (Fig.  56).  The  bending  of  the  head,  already  begun  in  the 
stage  last  studied,  has  continued  until  the  fore-  and  hind-brains  are 
nearly  parallel.  This  marked  cephalic  flexure  occurs  at  the  region  of  the 
mid-brain.  As  long  as  the  embryo  retained  its  original  prone  position 
with  respect  to  the  yolk,  it  is  manifest  that  the  head  could  not  bend 


EMBRYO  OF  TWENTY-SEVEN  SEGMENTS  (tWO  DAYS) 


337 


greatly  ventrad,  so,  in  order  that  the  flexion  might  proceed  to  completion, 
the  upper  body  has  twisted  about  its  long  axis  until  the  left  side  lies 
squarely  next  the  yolk.  In  a dorsal  view,  therefore,  one  sees  the  right 
side  of  the  head  but  the  dorsal  side  of  the  lower  body.  The  actual  zone 
of  torsion,  now  half  way  down  the  trunk,  will  advance  progressively  until 
the  whole  embryo  is  turned,  after  which  additional  curvatures  will  make  it 
assume  the  shape  of  the  letter  C (Fig.  363).  One  of  these  flexures  is 
already  appearing  opposite  the  lower  end  of  the  heart,  at  the  junction  of 
head  and  trunk;  for  this  reason  it  bears  the  name  cervical  flexure. 

Most  of  the  body  is  rather  sharply  delimited  from  the  blastoderm; 
the  head  is  free;  much  of  the  midbody  is  bounded  by  deep  lateral  folds; 
caudally,  the  tail  bud  represents  the  future  hind  end  of  the  body  and  is 
bordered  by  a tail  fold.  The  further  combined  activities  of  head-,  lateral-, 
and  tail  folds  will  constrict  the  embryo  from  the  extra-embryonic 
blastoderm. 

The  head  is  now  covered  by  a double  fold  of  the  somatopleure,  the 
head  fold  of  the  amnion;  it  envelops  the  upper  half  of  the  body  like  a veil. 
The  heart  bends  in  the  form  of  a letter  S,  and  the  extra-embryonic  vascular 
ple.xus  is  profuse.  Three  ectodermal  furrows  form  branchial  grooves  on 
the  sides  of  the  neck.  Eye  and  ear  anlages  are  prominent.  Primitive 
segments  extend  far  down  the  former  segmental  zone. 

Central  Nervous  System  and  Sense  Organs. — The  brain  region  of 
the  neural  tube  is  separated  by  constrictions  into  five  vesicles.  The  first 
subdivision  of  the  primitive  fore-brain  is  the  telencephalon;  the  rest  con- 
stitutes the  diencephalon.  The  mesencephalon  remains  undivided  but  is 
bent  at  its  middle  by  the  cephalic  flexure.  The  hind-brain  shows  two 
indistinct  regions  of  differentiation;  a short  section  with  a thick  roof 
adjoining  the  mid-brain  is  the  metencephalon,  the  thin-roofed  remainder 
is  the  myelencephalon.  The  spinal  cord  is  now  closed  to  its  extreme  end 
and  consequently  the  rhomboidal  sinus  no  longer  exists. 

The  lens  anlage  has  assumed  the  form  of  a lens  vesicle;  coincident  with 
its  invagination  the  outer  wall  of  the  optic  vesicle  also  folds  inward, 
thereby  making  a double-walled  structure,  the  optic  cup.  The  latter  is 
not  a complete  cup,  for  on  one  side  a segment  of  the  wall  is  missing;  this 
chorioid  fissure  gives  the  cup  a horse-shoe  outline  in  surface  view  (Fig. 
347).  The  auditory  placode  of  earlier  stages  has  become  a sac,  the 
otocyst  or  otic  vesicle,  which,  however,  retains  a connection  with  the  body 
ectoderm. 

Digestive  System. — The  entodermal  canal  shows  three  regional 
divisions.  Of  these,  the  fore-gut  is  best  differentiated  and  will  be  referred 
to  again.  In  Fig.  348  most  of  the  entoderm  has  been  removed,  so  that 
the  open  mid-gut  scarcely  shows;  it  extends  from  the  intestinal  portal  to 


33^ 


THE  STUDY  OF  CHICK  EMBRYOS 


the  tail  bud,  and,  without  a ventral  wall,  overlies  the  yolk.  Caudad,  a 
small  portal  leads  into  the  hind-gut  which  is  just  beginning  to  invaginate 
into  the  tail  fold;  in  development  and  relations  it  duplicates  the  fore- gut. 
The  pharyngeal  membrane  now  lies  at  the  bottom  of  a deep  pit,  the 
stomodeum,  formed  by  depressed  ectoderm.  A median  ectodermal  sac, 
just  in  front  of  the  pharyngeal  membrane  and  lying  next  the  brain  wall, 
is  Rathke's  pouch;  it  is  the  anlage  of  the  epithelial  portions  of  the  hypophy- 
sis. The  entodermal  pharynx  bears  three  pairs  of  lateral  outpocketings. 


Hind-brain 
Otic  vesicle 
Branch  til  groove  3 
Amnion  fold 


R.  vitelline  artery 
24 

rea  pellucida 


id-brain 


Fore-brain 
Optic  cup 
Lens  vesicle 


I ’( utricle  of  heart 
\ itcUine  vein 


Primitive  streak 
and  tail  bud 


L.  ‘Vitelline  artery 


Neural  tube 


Fig.  347. — Dorsal  view  of  a chick  embryo  with  twenty-seven  segments  (two  days). 


X 14. 


known  as  the  pharyngeal  pouches.  They  occur  opposite  the  three  external 
branchial  grooves,  and  here  ectoderm  and  entoderm  are  in  contact, 
forming  closing  plates.  At  about  this  stage  the  first  pair  of  plates  ruptures, 
thereby  making  a free  opening,  or  branchial  cleft,  into  the  pharynx.  These 
transitory  apertures  correspond  to  the  gill  clefts  in  lower  aquatic  verte- 
brates. Between  the  successive  pouches  lie  solid,  bar-like  portions  of  the 
body  wall,  the  branchial  arches;  in  animals  with  aquatic  respiration  the 
arches  bear  gills,  and  even  in  higher  embryos,  like  the  chick,  an  aortic 


EaiBRYO  OF  T\VEXTY-SEVEN  SEGMENTS  (XWO  DAYS) 


339 


arch  courses  through  each  (cf.  Fig.  184).  At  the  level  of  the  second  pair 
of  pouches,  a broadly  open  pocket  grows  from  the  median  floor  of  the 
pharynx;  it  is  the  anlage  of  the  thyroid  gland  (Fig.  353).  Toward  the 
intestinal  portal  the  fore-gut  is  flattened  from  side  to  side,  and  before 
it  opens  out  into  the  mid-gut  there  is  budded  off  a bilobed  liver  diverticu- 


350  

351  

352—— 

353 

354  

355  

356  

357  

358  


359 


360 

361 


Optic  cup 

A pert  lire  of  lens  vesicle 


Fore-brain 


Mid-brain 


Hind-brain 
Notochord 


Pharynx 

Bulb  of  heart 
Ventricle 


R.  vitelline  vein 
Fore-gut 

Splanchnopleure 
Splanchnic  mesoderm 

Dorsal  aorta 


R.  vitelline  artery 


Mesodermal  segment 


Segmental  zone 
Neural  plate  / 

\ 

Entoderm  \ 
Primitive  knot  K 


Otocyst 

Aortic  arches  1,2,3 

Ant.  cardinal  vein 
.itrium 

Common  cardinal  vein 

Post  cardinal  vein 
•^Descending  aorta 

Liver  anlage 
‘Intestinal  portal 

Entoderm 
Somatopleure 
Spinal  cord 


L.  vitelline  artery 
Edge  of  splanchnic  mesode 
Mesoderm  segment 
Vascular  plexus 

Notochord 

Hind-gut 


■350 

351 

-352 

-353 

354 

-355 


■356 


■357 


•358 


■359 

360 

■361 


Fig.  348. — Ventral  reconstruction  of  a twenty-seven  segment  chick  enbryo.  X 18. 
The  ectoderm  of  the  upper  body  and  the  entoderm  of  the  lower  body  have  been  mostly  removed. 
Numbered  lines  indicate  the  levels  of  Figs.  350-361. 


lum  (Figs.  348  and  355).  It  lies  between  the  vitelline  veins  which  later 
break  up  into  the  sinusoidal  spaces  of  the  liver. 

Vascular  System. — The  disappearance  of  the  dorsal  mesocardium 
leaves  the  huge,  tubular  heart  attached  only  at  its  two  ends.  Since  the 
heart  tube  is  growing  faster  than  the  surrounding  body,  it  of  necessity 


340 


THE  STUDY  OF  CHICK  EMBRYOS 


bends  like  the  letter  S,  when  seen  from  the  ventral  side  (Fig.  348).  Four 
regions  may  be  distinguished:  (i)  the  sinus  venosus,  into  which  the  veins  ; 
open;  (2)  a dilated  dorsal  chamber,  the  atrium;  (3)  a tubular  ventral 
portion,  bent  in  the  form  of  a U,  of  which  the  left  limb  is  the  ventricle,  the 
right  limb  (4)  the  bulbus  cordis.  From  the  bulbus  is  given  off  the  ventral  ;! 
aorta.  There  are  now  three  pairs  of  aortic  arches  which  open  into  the 
paired  descending  aorta’.  The  first  aortic  arch  extends  through  the  first 
branchial  arch,  cranial  to  the  first  pharyngeal  pouch,  and  is  the  primitive 
connecting  vessel  seen  in  the  thirty-eight-hour  embryo  (cf.  Fig.  184). 
The  second  and  third  aortic  arches  course  in  the  second  and  third  branchial 
arches  on  either  side  of  the  second  pharyngeal  pouch.  They  are  developed 
by  the  enlargement  of  channels  in  primitive  capillary  networks  between 
ventral  and  descending  aortre.  Opposite  the  sinus  venosus,  the  paired 
aortic  trunks  fuse  to  form  the  single  dorsal  aorta  which  extends  as  far 
back  as  the  fifteenth  pair  of  primitive  segments.  At  this  point  the  aortae 
again  separate,  and,  o])posite  the  twentieth  segments,  each  connects 
with  the  trunk  of  a vitelline  artery  which  conveys  blood  to  the  vascular 
area  (Fig.  348).  Caudal  to  the  vitelline  arteries,  the  aortas  decrease 
rapidly  in  size  and  soon  end. 

As  in  the  previous  stage,  the  blood  is  returned  from  the  vascular 
area  to  the  heart  by  the  vitelline  veins,  now  two  large  trunks  (Fig.  348).  ; 

In  the  body  of  the  embryo,  the  anterior  cardinal  veins  course  ventro- 
lateral to  the  brain  and  already  are  of  large  size.  The  smaller  posterior 
cardinal  veins  are  developing  caudal  to  the  atrium.  They  lie  in  the  mesen- 
chyme of  the  somatopleure,  laterad  in  position  ( Fig.  355).  Opposite  the 
sinus  venosus,  the  anterior  and  posterior  cardinal  veins  of  each  side  unite 
and  form  the  common  cardinal  veins  (ducts  of  Cuvier)  which  open  into  the 
dorsal  wall  of  the  sinus  venosus  (Fig.  348).  The  primitive  veins  are  thus 
paired  like  the  arteries,  and  like  them  develop  by  the  enlargement  of 
channels  in  a network  of  capillaries. 

Differentiation  of  Mesoderm. — The  formation  of  new  mesodermal 
segments  and  the  progressive  differentiation  of  older  ones  into  sclero- 
tome, myotome,  and  dermatome  continue  as  described  for  the  preceding 
embryo  (p.  330). 

The  ne])hrotome  region  shows  the  beginning  of  additional  features. 
The  prone phric  duct  has  continued  beyond  its  original  site  of  formation, 
and  as  a blind,  growing  cord  extends  tailward  (Fig.  336).  A set  of  new 
mesonephric  tubules  is  now  starting  to  differentiate,  caudal  to  the  pro- 
nephric  group,  between  the  thirteenth  and  thirtieth  segments.  They 
arise  from  the  intermediate  cell  masses  as  vesicles  that  will  become  tubules 
and  join  the  pronephric  (hereafter  called  mesonephric)  duct.  They 
constitute  the  embryonic,  but  not  the  definitive  kidney.  Additional 


EMBRYO  OF  TWENTY-SEVEN  SEGMENTS  (TWO  DAYS)  341 

and  mesonephroi  may  be  found  on  pp. 


information  concerning  the  pro- 

135-139- 

The  splanchnopleure  is  chiefly  involved  in  gut  formation.  The 
somatopleure  is  deeply  folded  into  the  lateral  body  folds  whose  union  will 
progressively  close  the  ventral  body  wall  (Fig.  356).  The  establishment 
of  a complete  body  wall,  at  any  level,  of  necessity  separates  embryonic 
from  extra-embryonic  coelom. 

Amnion  and  Chorion. — At  the  end  of  the  second  day,  two  extra-embry- 
onic, protective  membranes  have  become  prominent.  They  are  theawnion, 
which  will  form  a membranous,  fluid-filled  sac  about  the  embryo  itself,  and 

.4  B 


Amnion  folds 


Chorion 


D 


Fig.  349. — Diagrams  illustrating  the  development  of  the  amnion,  chorion  and  allantois 
in  longitudinal  section  (after  Gegenbauer  in  McMurrich).  Ectoderm,  mesoderm  and  entoderm 
are  represented  by  heavy,  light  and  dotted  lines  respectively.  .4/.,  Allantois;  Am.,  amniotic 
cavity;  lA.,  yolk  sac. 


the  chorion  which  eventually  encloses  both  embryo  and  all  extra-embryonic 
structures  (Fig.  37).  The  two  membranes  arise  simultaneously  from  the 
extra-embryonic  somatopleure  by  a single  process  of  folding  (Fig.  349). 
In  front  of  the  embryo  a fold  of  the  somatopleure  is  thrown  up,  followed 
later  by  others  lateral  and  caudal  to  the  embryo  (A).  These  hood-like, 
arching  folds  close  in  from  all  sides  until  they  meet  and  fuse  over  the 
embryo  (B-D).  The  inner  layer  of  somatopleure  is  the  amnion;  the 
remainder  constitutes  the  chorion,  of  little  importance  to  the  chick.  It 
should  be  noted  that  the  folding  brings  the  mesodermal  components  of 
these  membranes  facing  each  other,  but  separated  by  the  extra-embryonic 
coelom. 


342 


THE  STUDY  OF  CHICK  EMBRYOS 


The  head  fold  of  the  amnion  had  begun  in  the  chick  of  the  previous  stage 
(Fig-  335) : at  the  end  of  the  second  day  it  is  continuous  along  a crescentic 
margin  with  the  lateral  folds,  and  envelops  the  upper  half  of  the  body  (Figs. 
347  and  356).  As  yet  the  tail  fold  has  scarcely  started. 

Transverse  Sections 

The  following  series  of  transverse  sections  from  a two-day  chick  shows 
the  fundamentally  important  structures;  they  are  equally  applicable  to 
the  study  of  embryos  between  45  hours  (23  segments)  and  60  hours  (32 


Fig.  350. — Transverse  section  through  the  eyes  and  first  aortic  arches  of  a twenty-seven- 

segment  chick  embryo.  X 50. 

segments).  The  sections  are  drawn  from  the  caudal  surface;  hence,  the 
left  side  of  the  embryo  is  at  the  reader’s  left.  The  precise  level  of  each 
significant  section  should  be  ascertained  with  respect  to  Figs.  347  and  348, 
as  has  been  indicated  for  this  series  along  the  margins  of  Fig.  348.  Since 
the  head  is  bending  rajiidly  during  the  last  hours  of  the  second  day, 
minor  variations  in  the  appearance  of  different  series  of  sections  are 
unavoidable;  this,  however,  is  chiefly  a question  of  what  particular 
structures  happen  to  appear  together  in  the  fore-brain  and  hind-brain 
portions  of  a section. 


EMBRYO  OF  TWENTY-SEVEN  SEGMENTS  (tWO  DAYS) 


343 


Sections  through  the  Cephalic  Flexure. — Due  to  the  flexed  brain,  the  first  sections 
encountered  pass  through  the  mesencephalon,  but  soon  the  hind-hrain  and  then  the 
diencephalon  are  included.  The  blood  vessels  seen  are  the  anterior  cardinals.  Presently 
the  brain  becomes  cut  twice  in  each  section;  the  myelencephalon  may  be  recognized 
always  by  its  thin  roof  and  by  its  close  association  with  the  notochord.  Note  that  in 
these  sections  through  the  bent  head,  progress  is  caudad  down  the  hind-brain  half  of  the 
section,  but  cephalad  toward  the  tip  of  the  fore-brain. 

Section  through  the  Eyes  and  First  Aortic  Arches  (Fig.  350). — The  section  passes 
cephalad  of  the  optic  stalks,  consequently  the  optic  vesicles  appear  unconnected  with  the 
fore-brain.  The  adjacent,  thickened  ectoderm  is  invagiiiated  to  form  the  anlages  of  the 
lens  vesicles.  The  thicker  wall  of  the  optic  vesicle,  next  the  lens  anlage,  will  give  rise 
to  the  nervous  layer  of  the  retina;  the  thinner  outer  wall  becomes  the  pigment  layer 


Fig.  351. — Transverse  section  through  the  optic  stalks  and  hypophysis  of  a twenty-seven- 

segment  chick  embryo.  X 50. 

of  the  retina.  Ventrad  in  the  section  are  the  wall  and  cavity  of  the  fore-brain,  dorsad  the 
myelencephalon  of  the  hind-brain  with  its  thin,  dorsal  ependymal  layer.  Between  the  brain 
vesicles,  on  either  side,  are  longitudinal  sections  of  the  first  aortic  arches,  and  lateral  to  the 
hind-brain  are  the  smaller,  paired  anterior  cardinal  veins,  which  convey  the  blood  from 
the  head  to  the  heart.  The  splanchnopleure  (that  is,  the  yolk  side  of  the  blastoderm)  is 
characterized  in  this  and  subsequent  sections  by  the  presence  of  blood  vessels  in  its  meso- 
dermal layer.  The  entire  head  is  enveloped  by  the  amnion;  the  chorion  passes  along  the 
right  side  of  the  head,  and,  continuing,  surrounds  both  embryo  and  yolk.  In  these  mem- 
branes the  mesodermal  components  face  each  other  across  the  extra-embryonic  ccelom. 


THE  STUDY  OF  CHICK  EMBRYOS 


,U4 


Section  through  the  Optic  Stalks  and  Hypophysis  (Fig.  351). — The  section  passes 
just  caudal  to  the  lens.  'I'he  optic  vesicles  are  connected  with  the  wall  of  the  fore-brain 
by  the  optic  stalks,  which  later  form  the  path  through  which  the  fibers  of  the  optic  nerve 
grow  from  the  retina  to  the  lirain;  sections  cut  in  this  plane  do  not  show  the  chorioid  fissure. 
Both  the  ventral  and  the  descending  aortce  are  seen  about  the  cephalic  end  of  the  pharynx. 
Between  the  ventral  wall  of  the  fore-brain  and  the  pharynx  is  an  invagination  of  the  ecto- 
derm. Rathke's  pouch,  which  will  become  the  epithelial  hypophysis;  a few  sections  farther 
along  it  o])ens  externally  into  the  stomodcum,  close  to  the  pharyngeal  membrane. 

Passing  caudad  in  the  series  a short  distance,  the  fore-brain  region  of  the  bent  head 
liecomes  isolated  from  the  body  and  soon  the  tip  of  the  head  is  reached. 

Section  through  the  Otocysts  and  Second  Aortic  Arch  (Fig.  352). — The  otic  vesicles 
arc  sectioned  caudal  to  their  apertures,  and  so  appear  as  clo.sed  sacs,  lateral  to  the  wall  of 
the  hind-l)iain.  The  cavity  of  the  pharynx  is  somewhat  triangular  and  its  dorsal  wall  is 
thin.  The  anterior  cardinal  veins  pass  between  the  otocysts  and  the  wall  of  the  hind-brain. 


Mesoderm  of  hulhus 


Endothelium  of  hulhus 


Fi('..  352. — Transverse  section  through  the  otocysts  and  second  aortic  arches  of  a twenty-seven- 

segment  chick  embryo.  X50. 


Ventral  to  the  pharynx,  the  bulbus  cordis  is  sectioned  obliquely  where  it  leaves  the  heart, 
and  at  this  level  gives  off  the  second  pair  of  aortic  arches  which  connect  dorsad  with  the 
descending  aorta.  Surrounding  the  bulbus  cordis  is  the  large  pericardial  cavity,  not  yet 
enclosed  by  the  body  wall.  The  student  should  note  that  in  the  sections  so  far  studied, 
the  mesenchyme  of  the  head  is  undifferentiated,  the  tissues  peculiar  to  the  adult  not  yet 
having  formed.  The  amnion  attached  to  the  right  side  of  the  embryo  is  folded.  This  is 
because  the  primitive  amnion  folds  fuse  over  the  original  dorsal  line,  regardless  of  the  turn- 
ing of  the  embryo;  consequently,  on  the  right  there  is  ‘slack.’ 


EMBRYO  OF  TYENTY-SEVEX  SEGMENTS  (tM'O  DAYS) 


345 


Section  through  the  Second  Pharyngeal  Pouches  and  Thyroid  Anlage  (Fig.  353). — 
As  this  section  is  taken  at  a level  between  the  second  and  third  aortic  arches,  the 
descending  aortae  and  heart  are  unconnected.  Tangential  shavings  have  been  cut  from  the 
walls  of  the  otocysts.  Extending  laterally  from  the  pharynx  are  the  second  pair  of  pharyn- 
geal pouches  which  have  already  come  in  contact  with  the  ectoderm  to  form  closi)ig  plates. 
A pocket-like  depression  in  the  midventral  floor  of  the  pharynx  represents  the  thyroid 
anlage;  later,  it  becomes  saccular  and  loses  its  connection  with  the  pharyngeal  entoderm. 
The  splanchnic  mesodermal  wall  of  the  heart  is  destined  to  give  rise  later  to  the  epi-  and 
myocardium.  A short  distance  caudad  in  the  series,  the  large,  looped  ventricle  is  met;  it  is 
not  attached  by  the  former  dorsal  mesocardium. 

Section  through  the  Sinus  Venosus  and  Common  Cardinal  Veins  (Fig.  354). — At 
this  level,  the  common  cardinal  trunk,  formed  by  the  union  of  anterior  and  posterior  cardi- 
nal veins,  opens  into  the  thin-walled  sinus  venosus.  The  sinus  receives  all  of  the  blood 
passing  to  the  heart  and  is  separated  from  the  larger  atrium  by  a slight  constriction  only. 


Hind-hrain 
Entoderm 
Ectoderm 
Ant.  cardinal  vein 

Descending  aorta 
Closing  plate 
Pharynx 
Thyroid  anlage 

Mesoderm 


Epi-myocardium  of  hulhiis 
Endothelium  of  biilbus 


Fig.  353. — Transverse  section  through  the  second  pharyngeal  pouches  and  thyroid  anlage  of 
twenty-seven-segment  chick  embryo.  X 50. 


The  descending  aortcB  have  united  to  form  the  single  dorsal  aorta.  On  either  side  of  the 
pharynx  are  subdivisions  of  the  coelom  which  will  form  the  pleural  cavities  when  the  lung 
buds  appear.  These  cavities  are  separated  from  the  pericardial  cavity  by  the  septum 
transversum  (anlage  of  diaphragm)  in  which  the  common  cardinal  veins  cross  to  the  sinus 
venosus.  Since  the  last  section,  the  myelencephalon  has  merged  into  the  spinal  cord,  and 
the  dermo-myotomes  of  the  first  mesodermal  segments  are  seen.  The  mesodermal  compo- 
nents of  the  amnion  folds  are  not  fused  at  this  or  subsequent  levels. 

Section  through  the  Liver  Anlage  (Fig.  355). — In  this  section,  the  fore-gut  is  flattened 
from  side  to  side  and  its  cavity  is  narrow.  Ventrad,  there  are  evaginated  from  the  ento- 
derm two  diverticula  which  constitute  the  earliest  anlage  of  the  liver.  On  either  side  are 


346 


THE  STUDY  OF  CHICK  EMBRYOS 


sections  of  the  vitelline  veins  (the  left  swinging  in  from  the  blastoderm),  on  their  way  to  the 
sinus  venosus  at  a higher  level  in  the  series.  This  primitive  liver  anlage  does  not  always 


Spinal  cord 

Mesodermal  segment 
(denno-myotome) 


Dorsal  aorta 
Mesenchyme 

Common  cardinal  vein 

Entoderm 

Myocardium 


Fore-gut 

Splanchnic  mesoderm 
Ccelom 

Atrium 

Endothelium  of  heart 


Fig.  354. — Transverse  section  through  the  sinus  venosus  and  common  cardinal  veins  of  a 
twenty-seven-segment  chick  embryo.  X 50. 


Spinal  cord 
Mesodermal  segment- 
Entoderm- 
Ca'lom 

Fore-gut- 
L.  vitelline  vein 

Splanchnic  mesoderm 


Posterior  cardinal  vein 

Splanchnic  mesoderm 
Liver  anlage 

Vitelline  vein 


Fig.  355. — Transverse  section  through  the  liver  anlage  of  a twenty-seven-segment  chick 

embryo.  X 50. 

appear  bilobed;  at  a slightly  later  stage  it  is  found  ventral  to  the  united  vitelline  veins  and 
a second  anlage,  more  cephalad  in  origin,  lies  dorsal  to  the  vein.  Note  the  intimate  relation 


EMBRYO  OF  TWENTY-SEVEN  SEGMENTS  (tWO  DAYS) 


347 


between  the  entodermal  epithelium  of  the  liver  and  the  endothelium  of  the  vitelline  veins. 
In  later  stages,  as  the  liver  anlages  branch,  there  is  a mutual  intergrowth  between  the  ento- 
dermal cells  constituting  the  liver  and  of  the  vascular  endothelium  of  the  vitelline  veins. 
Thus  are  formed  the  hepatic  sinusoids  of  the  portal  system,  which  surround  the  cords  of 
hepatic  cells. 

The  septum  transversum  is  still  present  at  this  level,  and  lateral  to  the  fore-gut  are  small 
body  cavities.  Lateral  to  the  body  cavities  appear  branches  of  the  posterior  cardinal  veins. 

Section  through  the  Open  Gut  and  Amnion  Folds  (Fig.  356). — The  intestine  is  now 
open  ventrad,  its  splanchnopleure  passing  directly  over  to  that  of  the  vascular  area.  The 
dorsal  aorta  is  again  divided  by  a septum  into  its  primitive  components,  the  right  and  left 


Fig.  356. — Transverse  section  through  the  open  gut  and  amnion  folds  of  a twenty-seven- 

segment  chick  embryo.  X 50. 


357. — Transverse  section  through  the  seventeenth  pair  of  mesodermal  segments  of  a 
twenty-seven-segment  chick  embryo.  X 50. 

descending  aortce.  Lateral  to  the  aortae  are  the  small  posterior  cardinal  veins.  The  coelom 
is  in  communication  with  the  extra-embryonic  body  cavity.  Deep  lateral  body  folds  of 
somatopleure  indicate  how,  by  their  ventral  union,  the  body  becomes  established  free  from 


Ccelom 


Entoderm 


THE  STUDY  OF  CHICK  EMBRYOS 


34S 


the  blastoderm.  The  folds  of  the  amnion  have  not  joined,  thus  leaving  the  amniotic  cavity 
open;  (some  variation  may  be  found  in  the  exact  level  where  this  condition  occurs).  In 
such  a section,  the  somatopleuric  components  of  the  amnion  and  chorion  are  easily  traced, 
and,  a few  sections  cephalad,  the  manner  of  union  of  the  two  folds  is  illustrated. 

Section  through  the  Seventeenth  Pair  of  Mesodermal  Segments  (Fig.  357). — The 
body  of  the  embryo  is  no  longer  rotated.  On  the  left  side  of  the  figure,  the  mesodermal 
segment  shows  a der mo -myotome  plate.  The  median  and  ventral  portion  of  the  segment  is 
being  converted  into  sclerotomic  mesenchyme.  On  the  right  side,  near  the  upper  angle  of 
the  coelom,  appears  a section  of  the  proncpliric  (mesonephric)  duct.  The  open  space 
above  it  is  the  posterior  cardinal  vein;  some  sections  show  the  median  nephrotome  tissue 
organizing  into  mesonephric  tubule  anlages.  The  embryonic  somatoplcure  is  arched  and 
will  form  the  future  ventro-lateral  body  wall  of  the  embryo.  The  lateral  infoldings  of  the 
somatoiileure  give  indication  of  the  later  approximation  of  the  ventral  body  walls,  by 
which  the  embryo  it  separated  from  the  underlying  layers  of  the  blastoderm. 

Section  through  the  Origin  of  the  Vitelline  Arteries  (Fig.  358). — At  this  level,  the 
embryo  is  more  flattened  and  simple  in  .structure,  as  at  higher  levels  in  earlier  embryos. 


Mesodermal  segment 
Nephrotome 
C adorn 


Spinal  cord 

Ectoderm 


csoniatopleure 


Mesonephric  duct 

Somatic  mesoderm 


Splanchnoplcitrc* 

Aorta  and  vitelline  Notochord 
artery 

Fig.  358. — Transverse  section  through  the  origin  of  the  vitelline  arteries  of  a twenty-seven- 

segment  chick  embryo.  X 50. 

Mesodermal  segments,  nephrotomes,  and  lateral  layers  of  somatic  and  splanchnic  meso- 
derm are  little  differentiated.  The  amniotic  folds  have  not  appeared.  On  the  left  side 
of  thehgure,  the  vitelline  artery  leaves  the  aorta;  on  the  right  side,  the  connection  of  the  vitel- 
line artery  with  the  aorta  does  not  show,  as  the  section  is  cut  somewhat  obliquely.  The 
posterior  cardinal  vein  is  present  just  laterad  of  the  right  mesonephric  duct.  The  small 
clusters  of  cells  dorso-lateral  to  the  spinal  cord  are  the  neural  crests  which  will  differentiate 
into  spinal  ganglia. 

Spinat  cord  Ectoderm 


Fig.  359. — Transverse  section  through  the  segmental  zone  of  a twenty-seven-segment  chick- 

embryo.  X 50. 

Section  through  the  Segmental  Zone  (Fig.  359). — The  mesodermal  segments  are 
replaced  by  the  segmental  zone,  a somewhat  triangular  mass  of  undifferentiated  mesoderm 
from  which  later  are  formed  the  segments  and  nephrotomes.  The  notochord  is  larger. 


EMBRYO  OF  THREE  TO  FOUR  DAYS 


349 


the  aorta  smaller,  and  a few  sections  caudad  they  disappear.  Laterally,  the  somatoplciire 
and  splanchnopleure  are  straight  and  separated  by  the  slit-like  ccelom. 

Section  through  the  Tail  Bud,  Cranial  to  the  Hind-gut  (Fig.  360). — With  the  exception 
of  the  ectoderm,  the  structures  near  the  median  plane  are  merged  into  an  undifferentiated 
mass  of  dense  tissue,  the  notochordal  plate.  The  cavity  of  the  closed  neural  tube  and  its 
dorsal  outline  may,  however,  still  be  seen.  Laterally,  the  segmental  zone  and  the  various 
layers  are  differentiated. 


Neural  tube  Ectoderm 


Fig.  360. — Transverse  section  through  the  tail-bud  of  a twenty-seven-segment  chick  embryo. 

X 50. 


Section  through  the  Hind-gut  and  Primitive  Streak  (Fig.  361). — In  this  embryo,  the 
caudal  evagination  to  form  the  hind-gut  has  just  begun.  The  section  shows  the  small 
cavity  of  the  hind-gut  in  the  midplane.  Its  wall  is  composed  of  columnar  entodermal  cells 
and  it  is  an  outgrowth  of  the  entodermal  layer.  A few  sections  cephalad  in  the  series,  the 
hind-gut  opens  by  its  own  posterior  intestinal  portal.  Dorsal  to  the  hind-gut  may  be  seen 
undifferentiated  cells  of  the  primitive  streak,  continuous  dorsad  with  the  ectoderm,  ventrad 
with  the  entoderm  of  the  hind-gut,  and  laterally  with  the  mesoderm. 


Primitive  streak 


Fig.  361. — Transverse  section  through  the  hind-gut  of  a twenty-seven-segment  chick  embryo. 

X 50- 

(F)  EMBRYOS  OF  THREE  TO  FOUR  DAYS 
During  the  third  and  fourth  days  of  incubation  the  chick  attains  a 
stage  of  development  corresponding  to  the  younger  of  the  pig  embryos 
customarily  studied.  It  is  advisable,  therefore,  to  describe  only  such 
essential  features  of  developmental  advance  in  these  older  chick  embryos 
as  are  necessary  for  introducing  the  detailed  pig  studies  which  follow. 

External  Form. — ^The  whole  body  shows  the  effect  of  torsion,  and  the 
embryo  now  lies  on  its  left  side  (Fig.  362).  The  former  flexures,  espe- 
cially the  cervical,  are  pronounced,  and  new  dorsal  and  caudal  flexures 
have  appeared;  as  a result,  the  embryo  becomes  so  curved  that  its  head  and 
tail  approach.  The  final  number  of  42  primitive  segments  is  present  and 


35° 


THE  STUDY  OF  CHICK  EMBRYOS 


the  body  ends  in  a distinct  tail.  Up]ier  and  lower  limb  buds  extend  from 
the  body  wall,  and  the  saccular  allantois  projects  through  the  unclosed 
lower  abdomen.  Four  branchial  clefts  show,  separated  by  prominent 
branchial  arches.  The  continued  undercutting  of  the  body  folds,  espe- 
cially the  more  recent  tail  fold,  has  reduced  the  area  of  attachment  with 
the  yolk  sac  to  a relatively  narrow  yolk  stalk  (Fig.  363). 

Central  Nervous  System  and  Sense  Organs. — The  five  secondary 
divisions  of  the  brain  are  easily  identified;  the  telencephalon  bears  lateral 


Branchial  cleft  2 Ganglion  N.  0 


hemispheres  and  the  distinction  between  metencephalon  and  myelen-  |' 
cephalon  is  now  plain.  Most  of  the  cranial  nerves  and  ganglia  have  begun  j 
to  appear  (Fig.  362).  From  the  roof  of  the  diencephalon  is  the  evagina-  ! 
tion  of  the  epiphysis,  in  its  floor  the  anlage  of  the  neural  lobe  of  the  hypo-  Jf 
physis  (Fig.  363).  j 

The  eye  is  a prominent  organ,  with  its  lens  freed  from  the  ectoderm  jf 
but  with  the  narrowed  chorioid  fissure  still  showing  (Fig.  362).  The  otic  ji 
vesicle  is  a detached,  closed  sac  from  which  the  tubular  endolym ph  duct  is 


EMBRYO  OF  THREE  TO  FOUR  DAYS 


351 


growing.  Olfactory  anlages,  not  seen  hitherto,  have  appeared  as  ecto- 
dermal placodes  on  the  ventro-lateral  sides  of  the  head;  they  are  now 
depressed  as  olfactory  pits. 

Digestive  and  Respiratory  Systems. — The  fore-gut  and  hind-gut  are 
complete  tubes,  and  the  open  mid-gut  is  a relatively  short  segment  con- 
nected by  the  yolk  stalk  with  the  yolk  sac  (Fig.  363).  As  the  pharyngeal 
membrane  has  ruptured,  the  stomodeum  becomes  an  integral  part  of  the 
mouth  cavity.  Y over  pharyngeal  pouches  are  prominent;  in  all  but  the  fourth, 
the  closing  plates  perforate  and  form  temporary  branchial  clefts.  At  this 


Notochord  Mandible 


time,  the  median,  thyroid  diverticulum  loses  connection  with  the  floor  of 
the  pharynx.  The  trachea  has  arisen  from  a midventral  groove  which 
separates  from  the  caudal  end  of  the  pharynx  and  bifurcates  into  two 
lung  buds.  The  esophagus  is  a short,  narrowed  segment  and  the  stomach 
a slightly  spindle-shaped  dilatation.  Both  liver  anlages  have  fused  into  a 
branching  mass  and  at  the  same  level  the  pancreas  is  appearing. 

Except  for  the  attachment  of  the  yolk  sac.  there  are  no  additional 
features  of  interest  above  the  caudal  end  of  the  hind-gut.  Here  the  gut  is 
separated  from  an  ectodermal  pit,  the  proctodeum,  by  a thin  cloacal  mem- 


352 


THE  STUDY  OF  CHICK  EMBRYOS 


hranc  which  later  perforates  (Fig.  363).  The  mesonephric  ducts  join  the 
hind-gut,  and  a stalked  vesicle,  the  allantois,  grows  from  its  ventral  floor. 
This  common  chamber,  which  receives  the  contents  of  the  intestine  and 
the  secretions  of  the  urinary  and  reproductive  glands,  is  the  cloaca. 

Urinary  System.-  -The  pronephric  tubules  disappear  on  the  fourth 
day.  Mesonephric  tubules  are  still  developing  and  consist  of  elongate,  | 
coiled  tubules  associated  with  a glomerulus  at  one  end  and  with  the 
mesonephric  duct  at  the  other.  The  metanephros,  or  permanent  kidney 
anlage,  is  just  appearing;  its  collecting  tubules  and  ureter  arise  as  a bud 
from  the  mesonephric  duct  near  the  cloaca;  the  secretory  tubules  will 
develop  from  caudal  nephrotome  tissue. 

Vascular  System. — The  ventricular  loop  has  moved  caudad  and  the 
atrial  region  cephalad,  thus  reversing  the  original  positional  relations  of 
these  jiarts  (Fig.  362).  Both  atrium  and  ventricle  show  external  indica- 
tions of  a beginning  division  into  right  and  left  chambers,  and  the  myocar- 
dial wall  is  assuming  the  characteristics  of  muscle  cells.  As  a whole,  the 
heart  has  sunk  caudad  considerably  from  its  early  cephalic  position. 

Below  the  heart,  the  primitive  aortae  are  fused  throughout  their 
lengths.  Since  the  second  day,  a fourth,  a rudimentary  fifth,  and  a . 
sixth  pair  of  arches  have  developed;  of  the  full  set,  only  the  third  (carotid), 
fourth  (aortic),  and  sixth  (pulmonary)  arches  remain.  The  cardinal  veins  i' 
are  well  developed,  and  the  paired  vitelline  arteries  and  veins  have  both  , 
fused  inside  the  body  into  single  vessels.  New  umbilical  arteries  pass  to 
the  allantois,  and  umbilical  veins  return  the  blood  by  way  of  the  lateral  i! 
body  wall  to  the  heart.  These  will  become  still  more  important  in  the 
mammal. 

Extra-embryonic  Membranes. — During  the  third  day  the  tail-fold  of 
the  amnion  develops,  and  soon  the  embryo  becomes  enclosed  by  the  fluid- 
filled  amnion  sac  which  is  protective  in  function  (Figs.  37  and  349);  the 
chorion,  formed  by  the  same  process,  but  of  little  significance,  ultimately 
surrounds  the  embryo  and  all  extra-embryonic  structures.  Much  of  the 
yolk  sac  is  covered  by  advancing  splanchnopleure  which  is  continuous 
over  a relatively  narrow  yolk  stalk  with  that  of  the  gut  (Fig.  363).  As  the 
embryo  elongates,  the  yolk  stalk  appears  relatively  narrower.  Through 
the  vitelline  vessels  the  yolk  supplies  all  the  food  material  for  embryonic 
growth.  The  allantois  arises  late  in  the  third  day  as  a diverticulum  of  the 
splanchnopleuric  floor  of  the  hind-gut  (Figs.  349  and  363).  It  later 
becomes  a large,  stalked  sac  occupying  the  space  beneath  the  shell. 
Umbilical  vessels  ramify  in  its  walls  and  the  allantois  serves  as  the  prin- 
cipal organ  of  respiration  and  excretion. 


CHAPTER  XVII 


THE  STUDY  OF  PIG  EMBRYOS 


A’ery  young  pig  embryos  of  the  primitive  streak  and  neural  fold 
stages  are  shown  in  Fig.  364.  The  closure  of  the  neural  tube  and  the 
progressive  appearance  of  mesodermal  segments  are  likewise  illustrated 
in  Fig.  365.  The  fundamental  similarity  of  these  embryos  to  the  early 
chicks  already  studied  is  apparent.  For  a short  time,  succeeding  stages 
are  complicated  by  flexion  and  spiral  twisting  which  make  sections  difficult 
for  the  beginner  to  interpret.  In  embryos  about  6 mm.  long,  the  twist  of 


Fig.  364. — Early  pig  embryos  (Keibel).  X 20.  A,  Blastoderm  with  primitive  streak  and 
knot;  B,  blastoderm  with  primitive  streak  and  neural  groove. 

the  body  has  disappeared  sufficiently  so  that  its  structure  may  be  studied 
to  better  advantage.  At  this  time  the  state  of  development  is  generally 
comparable  to  that  of  a four  day  chick  (Fig.  366). 

The  fetal  membranes  of  the  pig  stand  somewhat  intermediate  between 
the  chick  and  man.  The  amnion,  chorion,  and  allantois  develop  very 
much  as  in  the  chick  (Fig.  349).  The  yolk  sac  is  small  and  rudimentary, 
so  its  functions  are  transferred  to  the  allantois  which  fuses  with  the  chorion ; 
the  two  constitute  a placenta  which  is  the  organ  of  fetal  respiration, 
nutrition,  and  excretion  (Fig.  39).  The  development  and  relations  of  these 
extra-embryonic  structures  are  described  on  pp.  46-49. 


B 


353 


354 


THE  STUDY  OF  PIG  EMBRYOS 


In  this  manual,  series  of  transverse  sections  are  figured  and  described 
only.  Lateral  and  sagittal  dissections  show  the  longitudinal  relations 
more  clearly  than  serial  sections  cut  in  these  planes;  if,  however,  sagittal 
or  frontal  sections  are  used,  they  may  be  interpreted  readily  from  the 
corresiionding  dissections  and  reconstructions. 


Fig.  365. — Dorsal  views  of  pig  embryos,  with  the  amnion  cut  away  (Keibel).  X 20.  A, 
Embryo  of  seven  segments;  B,  embryo  of  eleven  segments. 

(A)  THE  ANATOMY  OF  A SIX  MM.  PIG  EMBRYO 

The  descriptions  given  here  are  applicable  to  the  study  of  embryos 
between  5 and  8 mm.  Due  to  a shorter  term  of  development,  a 6 mm. 
pig  embryo  is  slightly  farther  advanced  in  most  respects  than  a human 
embryo  of  the  same  size  (Fig.  61). 

External  Form. — Both  head  and  body  are  bent  in  an  even  curve, 
convex  along  the  dorsal  line,  and  the  tail  is  recurved  sharply  (Fig.  366). 
The  cephalic  flexure  forms  an  acute  angle  at  the  mesencephalon,  and  there 
is  also  a marked  cervical  flexure.  As  a result,  the  head  is  somewhat  trian- 
gular in  shape.  Lateral  to  the  dorsal  line  may  be  seen  the  segments, 
which  become  larger  and  more  differentiated  toward  the  head.  At  the  tip 
of  the  head,  a shallow  depression  marks  the  olfactory  pit.  The  lens  vesicle 
of  the  eye  is  open  to  the  exterior.  Caudal  to  the  eyes,  at  the  sides  of  the 
head,  are  four  branchial  arches  separated  by  three  branchial  grooves.  The 


THE  ANATOMY  OF  A SIX  MM.  PIG  EMBRYO 


355 


fourth  arch  is  partly  concealed  in  a triangular  depression,  the  cervical  sinus, 
formed  by  the  more  rapid  growth  of  the  first  and  second  arches  (cf.  Fig. 
369).  The  first,  or  mandibular  arch,  forks  ventrally  into  two  parts,  a 
smaller  maxillary-  and  a larger  mandibular  process;  the  latter,  with^its. 
fellow,  forms  the  lower  jaw.  The  position  of  the  mouth  is  indicated  by 
the  space  between  these  processes.  The  furrow  from  the  eye  to  the  mouth 
is  the  lacrimal  groove.  The  second,  or  hyoid  arch  is  separated  from  the 
mandibular  arch  by  an  ectodermal  groove  which  persists  as  the  external 
acoustic  meatus. 


Maxillary  process  Mandibular  process 


The  heart  is  large,  and  through  the  transparent  body  wall  may  be 
seen  the  dorsal  atrium  and  ventral  ventricle.  Caudal  to  the  heart,  a 
convexity  indicates  the  position  of  the  liver,  dorsal  to  which  is  the  bud  of 
the  upper  limb.  Extending  caudad  from  the  limb  bud,  a curved  convexity 
indicates  the  position  of  the  left  mesonephros,  large  and  precociously 
developed  in  the  pig.  At  its  end  is  the  anlage  of  the  lower  limb.  The 
amnion  has  been  dissected  away  along  the  line  of  its  attachment,  ventral 
to  the  mesonephros.  There  is  as  yet  no  distinct  umbilical  cord,  and  a 
portion  of  the  body  stalk  is  attached  to  the  embryo. 

Nervous  System  and  Sense  Organs. — The  brain  is  differentiated  into 
its  five  regions;  telencephalon,  diencephalon,  mesencephalon,  metenceph- 
alon,  and  myelencephalon  (Fig.  367).  The  spinal  cord  is  cylindrical 
and  tapers  off  gradually  to  the  tail.  The  anlage  of  the  cranial  and  spinal 
ganglia  and  the  main  nerve  trunks  are  shown.  The  olfactory,  optic,  and 


356 


THE  STUDY  OF  PIG  EMBRYOS 


trochlear  nerves  have  not  yet  differentiated,  and  the  oculomotor  nerve  is 
just  beginning  to  appear  from  the  ventral  wall  of  the  mesencephalon. 
Ventrodateral  to  the  metencephalon  and  myelencephalon  occur  in  order: 
the  semilunar  ganglion  and  three  branches  of  the  trigeminal  nerve;  the 
geniculate  ganglion  and  nerve  trunk  of  the  n.  facialis;  the  ganglionic  anlage 
of  the  11.  acusticus.  Caudal  to  the  otocyst,  a continuous  chain  of  cells 
extends  lateral  to  the  neural  tube  into  the  tail  region.  Cellular  enlarge- 


ments along  this  neural  crest  represent  developing  cranial  and  spinal 
ganglia.  They  are,  in  order:  the  superior,  or  root  ganglion  of  the  glosso- 
pharyngeal nerve  with  its  distal  petrosal  ganglion;  the  ganglion  jugulare  and 
distal  ganglion  nodosum  of  the  vagus  nerve;  the  ganglionic  crest  and  the 
proximal  portion  of  the  spinal  accessory  nerve;  and  the  anlage  of  Froriep’s 
ganglion,  an  enlargement  on  the  neural  crest  just  cranial  to  the  first 
cervical  ganglion.  Between  the  vagus  and  Froriep’s  ganglion  may  be 
seen  the  numerous  root  fascicles  of  the  hypoglossal  nerve,  which  take  their 


THE  ANATOMY  OF  A SIX  MM.  PIG  EMBRYO 


357 


origin  along  the  ventro-lateral  wall  of  the  myelencephalon  and  unite  to 
form  a single  trunk.  The  posterior  roots  of  the  spinal  ganglia  are  very 
short ; their  anterior,  or  ventral  roots  are  concealed.  It  should  be  observed 
that  the  fifth,  seventh,  ninth,  and  tenth  cranial  nerves  pass  to  the  four 
branchial  arches  in  the  order  named.  This  primitive  relation  is  main- 
tained in  the  adult  when  the  nerves  innervate  the  derivatives  of  these 
arches. 

The  olfactory  pits  are  merely  slight  depressions  in  the  thickened  ecto- 
derm of  the  ventral  head.  There  are  stalked  but  the resides 

still  open  to  the  exterior.  The  otocysts  are  oval,  ectodermal  vesicles 
with  endolymph  ducts  just  appearing  as  dorso-mesial  outgrowths. 

Digestive  and  Respiratory  Systems. — The  month  lies  between  the 
mandible,  the  median  fronto-nasal  process  of  the  head,  and  the  maxillary 
processes  at  the  sides  (Fig.  369).  The  diverticulum  of  the  epithelial 
hypophysis  {Rathke's  pouch)  extends  along  the  ventral  wall  of  the  fore- 
brain (Fig.  368) ; near  its  distal  end,  the  wall  of  the  brain  is  thickened,  and 
later  the  posterior  lobe  of  the  hypophysis  will  develop  at  this  point. 

The  pharynx  is  flattened  dorso-ventrally  and  is  widest  near  the  mouth. 
It  narrows  caudad,  and,  opposite  the  third  branchial  arch,  makes  an  abrupt 
bend,  which  corresponds  to  the  cervical  flexure  of  the  embryo’s  body  (Fig. 
368).  In  the  roof  of  the  phar^mx,  just  caudal  to  Rathke’s  pouch,  is  the 
somewhat  cone-shaped  pocket  known  as  SeesseV s pouch,  which  may  be 
interpreted  as  the  blind,  cephalic  end  of  the  fore-gut  (Fig.  376).  The 
lateral  and  ventral  walls  of  the  pharynx  and  oral  cavity  are  shown  in  Fig. 
S4  A.  Of  the  four  arches,  the  mandibular  is  the  largest,  and  a groove  partly 
separates  the  tongue  anlages  of  the  two  sides.  Posterior  to  this  groove  and 
extending  in  the  median  plane  to  the  hyoid  arch  is  a triangular  elevation, 
the  tuberculmn  impar;  it  later  vanishes,  apparently  contributing  nothing 
to  the  tongue.  At  an  earlier  stage,  the  median  thyroid  anlage  grew  out 
from  the  midventral  wall  of  the  pharynx  just  caudal  to  the  tuberculum 
impar.  The  ventral  ends  of  the  second  arches  fuse  in  the  midventral 
plane  and  form  a prominence,  the  copula  (Fig.  84  B).  This  connects  the 
tuberculum  impar  with  a rounded  tubercle  derived  from  the  third  and 
fourth  pairs  of  arches,  the  anlage  of  the  epiglottis.  Its  cephalic  portion 
forms  the  root  of  the  tongue.  Caudal  to  the  epiglottis  are  the  arytenoid 
ridges,  and  a slit  between  them,  the  glottis,  leads  into  the  trachea. 

The  branchial  arches  converge  caudad,  and  the  pharynx  narrows 
rapidly  before  it  is  differentiated  into  the  trachea  and  esophagus.  Later- 
ally and  ventrally,  between  the  arches,  are  the  four  paired  outpocketings  of 
the  pharyngeal  pouches  (Fig.  375).  The  pouches  have  each  a dorsal  and 
ventral  diverticulum.  The  dorsal  diverticula  are  large  and  wing-like; 
they  meet  the  ectoderm  of  the  branchial  grooves  and  fuse  with  it  to  form 


358 


THE  STUDY  OF  PIG  EMBRYOS 


the  closing  plates.  Between  the  ventral  diverticula  of  the  third  pair  of 
pouches  lies  the  median  thyroid  anlage  (Fig.  376).  The  fourth  pouch  is 
smaller  than  the  others;  its  dorsal  diverticulum  just  meets  the  ectoderm; 
its  ventral  portion  is  small,  tubular  in  form,  and  is  directed  parallel  to 
the  esophagus. 

The  groove  on  the  floor  of  the  pharynx,  caudal  to  the  epiglottis,  is 
continuous  with  the  tracheal  groove.  More  caudally,  opposite  the  atrium 


of  the  heart,  the  trachea  has  separated  from  the  esophagus  (Fig.  368).  The 
trachea  at  once  bifurcates  to  form  the  primary  bronchi  and  the  anlages  of 
the  lungs  (Fig.  369).  The  latter  consist  merely  of  the  dilated  ends  of  the 
bronchi,  surrounded  by  a layer  of  splanchnic  mesoderm.  They  bud  out 
laterally  on  each  side  of  the  esophagus  near  the  cardiac  end  of  the  stomach, 
and  project  into  the  pleural  coelom.  The  esophagus  is  short,  and  widens 


limh  of  intestine 


Spinal  cord 

R.  mesonephros  Mesonephric  duct 
Fig.  368. — Median  sagittal  dissection  of  a 6 mm.  pig  embryo.  X 18 


Anlage  of 
tongue 


R.  atrium 


Mesencephalon 


Isthmus 


Esophagus 

Interatrial 
fora  men 
Lung  hud 

Stomach 


Gall  bladder 


V.  pancreas 
(at  left, 

D.  pancreas) 


Cranial  limb 
of  intestine 


L.  gen  Hat  fold 


Diencephalon 


Bulbils  cordis 
Telencephalon 
Ventricle 


Septum  trans- 
versum 

Liver 

Yolk  sac 

Allantois 

Tail  gut 

Cloaca 

Metanephros 


I'harynx 


Neuromere  4 Rathke's  pouch 


THE  ANATOMY  OF  A SIX  MM.  PIG  EMBRYO 


359 


dorso-ventrally  to  form  the  stomach.  The  long  axis  of  the  stomach  is 
nearly  straight,  but  its  entodermal  walls  are  compressed  and  it  has 
revolved  on  its  long  axis  so  that  the  original  dorsal  border  lies  to  the  left, 
the  ventral  border  to  the  right  (Fig.  382). 

Caudal  to  the  pyloric  end  of  the  stomach,  and  to  its  right,  is  given  off 
from  the  duodenum  the  hepatic  diverticulum  (Fig.  368).  This  is  a sac  of 
elongated  oval  form  from  which  the  liver  and  part  of  the  pancreas  take 
origin,  and  which  later  gives  rise  to  the  gall  bladder,  cystic  duct,  and  com- 
mon bile  duct.  It  is  connected  by  several  cords  of  cells  with  the  trabeculae 


of  the  liver;  the  latter  is  divided  incompletely  into  four  lobes,  a small 
dorsal  and  a large  ventral  lobe  on  each  side  (Fig.  367). 

The  pancreas  is  represented  by  two  outgrowths.  The  ventral  pancreas 
originates  from  the  hepatic  diverticulum  near  its  attachment  to  the  duo- 
denum (Fig.  368).  It  grows  to  the  right  of  the  duodenum  and  ventral  to 
the  portal  vein.  The  dorsal  pancreas  takes  origin  from  the  dorsal  side 
of  the  duodenum,  caudal  to  the  hepatic  diverticulum,  and  grows  dorsally 
into  the  substance  of  the  gastric  mesentery  (Fig.  376).  It  is  larger  than 


360 


THE  STUDY  OF  PIG  EMBRYOS 


the  ventral  pancreas,  and  its  posterior  lobules  grow  to  the  right  and  dorsal 
to  the  portal  vein,  and  in  later  stages  anastomose  with  the  lobules  of  the 
ventral  pancreas. 

The  intestine  of  both  fore-gut  and  hind-gut  has  elongated  and  curves 
ventrally  into  the  region  of  the  future  umbilical  cord  wdiere  the  yolk 
stalk  has  nari'ow^ed  at  its  point  of  attachment  to  the  gut  (Fig.  368).  The 
cloaca,  a dorso-ventrally  expanded  portion  of  the  hind-gut,  gives  off 
cephalad  and  ventrad  the  allantoic  stalk.  This  is  at  first  a narrow  tube, 
but  soon  expands  into  a vesicle  of  large  size,  a portion  of  which  is  seen  in 
Fig.  368.  Dorso-laterad,  the  cloaca  receives  the  mesonephric  ducts. 
The  hind-gut  is  continued  into  the  tail  as  the  transitory  tail- gut,  or  post- 
anal  gut,  which  dilates  at  its  extremity.  The  midventral  wall  of  the 
cloaca  is  fused  to  the  adjacent  ectoderm  to  form  the  cloacal  membrane. 
In  this  region  the  anus  will  appear. 

Coelom  and  Mesenteries. — The  coelom  communicates  throughout, 
but  already  consists  of  a single,  large  pericardial  cavity,  paired  pleural 
canals,  and  a common  peritoneal  chamber.  The  septum  transversum, 
which  will  form  most  of  the  diaphragm,  is  prominent  and  serves  to  sep- 
arate partially  the  heart  cavity  from  the  remainder  of  the  coelom  (Fig. 
368).  The  primitive  dorsal  mesentery  is  a thick,  double  layer  of  splanchnic 
mesoderm  investing  the  gut  and  attaching  it  to  the  median  roof  of  the 
peritoneal  cavity.  As  the  intestine  bends  out  toward  the  yolk  sac,  the  dor- 
sal mesentery  grows  at  an  equal  rate  and  suspends  it  (Fig.  367).  The 
liver  lies  in  the  ventral  mesentery,  between  the  stomach-duodenum  and  the 
midventral  body  wall.  Between  this  level  and  the  yolk  stalk,  the  mesentery 
is  beginning  to  disappear;  the  caudal  limb  of  the  intestine  is  already  free 
ventrad  (Fig.  376). 

Urogenital  System. — The  form  of  the  mesonephroi  is  seen  in  Figs. 
367  to  369.  Each  consists  of  large,  vascular  glomeruli,  associated  with 
coiled  tubules  which  are  lined  with  cuboidal  epithelium  and  open  into  the 
mesonephric  duct  (Fig.  13 1).  The  mesonephric  {Wolffian)  duct,  beginning 
at  the  anterior  end  of  the  mesonephros,  curves  at  first  along  its  ventral, 
then  along  its  lateral  surface.  At  the  caudal  end,  each  duct  bends  ven- 
trad and  to  the  midplane  where  it  opens  into  a lateral  expansion  of  the 
cloaca  (Fig.  368).  Before  this  junction  takes  place,  an  evagination  into  the 
mesenchyme  from  the  dorsal  wall  of  each  mesonephric  duct  gives  rise  to 
the  anlage  of  the  mctanephros,  or  permanent  kidney.  The  allantois  is  a 
prominent,  stalked  sac  communicating  with  the  ventral  part  of  the  cloaca. 
A slight  thickening  of  the  mesothelium  along  the  median  and  ventral 
surface  of  each  mesonephros  forms  a light-colored  area,  the  genital  fold 
(Fig.  368).  This  ridge  is  pointed  at  either  end  and  confined  to  the  middle 
third  of  the  kidney.  It  is  the  anlage  of  the  genital  gland. 


THE  ANATOMY  OE  A SIX  MM.  PIG  EMBRYO 


361 


Vascular  System. — The  heart  lies  in  the  pericardial  cavity,  as  seen 
in  Fig.  368.  The  atrial  region  (Fig.  370),  now  includes  two  lateral  sacs, 
the  right  and  left  atria.  Similarly,  the  bulbo-ventricular  loop  has  become 
differentiated  into  right  and  left  ventricles,  much  thicker  walled  than  the 
atria.  The  right  ventricle  is  the  smaller,  and  from  it  the  hulhiis  passes 
between  the  atria  and  is  continued  as  the  ventral  aorta.  Viewed  from  the 
caudal  and  dorsal  aspect  (Fig.  371),  the  sinus  venosiis  is  seen  dorsal  to 
the  atria.  It  opens  into  the  right  atrium  and  receives  from  the  right  and 


Fig.  370. — Ventral  and  cranial  surface  of  the  heart  from  a 6 mm.  pig  embryo.  X 14. 

left  sides  the  paired  common  cardinal  veins.  These  veins  drain  the  blood 
from  the  body  of  the  embryo.  Caudally,  the  sinus  venosus  receives  the 
two  vitelline  veins.  Of  these,  the  left  is  small  in  the  liver  and  later  dis- 
appears. The  right  vitelline  vein,  now  the  common  hepatic,  carries  most  of 


Fig.  371. — Dorsal  and  caudal  view  of  the  heart  from  a 6 mm.  pig  embryo.  X ^i. 

the  blood  to  the  heart  from  the  umbilical  veins,  and  from  the  liver  sinu- 
soids, gut,  and  yolk  sac. 

Transverse  sections  of  the  embryo  through  the  four  chambers  of  the 
heart  show  the  atria  in  communication  with  the  ventricles  through  the 
atrio-ventricular  foramina,  and  the  sinus  venosus  opening  into  the  right 
atrium  (Fig.  380).  This  opening  is  guarded  by  the  right  and  left  valves  of 
the  sinus  venosus.  Septa  partition  the  two  atria  and  the  two  ventricles 
incompletely.  In  Fig.  380  the  atrial  septum  {septum  primum)  appears 


362 


THE  STUDY  OF  PIG  EMBRYOS 


complete,  due  to  the  plane  of  the  section,  but  in  Fig.  372,  from  a slightly 
smaller  embryo,  it  is  seen  that  the  septum  primum  grows  from  the  dorsal 
atrial  wall  of  the  heart  and  does  not  yet  meet  the  endocardial  cushions 
between  the  atrio-ventricular  canals.  This  opening  between  the  atria 
is  known  as  the  interatrial  foramen.  Before  it  closes,  another  aperture 
appears  in  the  septum,  dorsal  in  position;  this  is  the  foramen  ovale  which 
persists  during  fetal  life.  In  Fig.  372  both  openings  may  be  seen,  as  may 
also  the  dorsal  and  ventral  endocardial  cushions  bounding  the  atrio-ventri- 
cular foramina.  The  mesothelial  layer  of  the  ventricles  has  become  much 
thicker  than  that  of  the  atria.  It  forms  the  epicardimn  and  the  myo- 
cardium; the  sponge-like  meshes  of  the  latter  are  now  being  developed. 

The  Arteries. — Beginning  with  the  ventral  aorta,  which  takes  origin 
from  the  bulbus  cordis,  pairs  of  aortic  arches  are  given  off.  These  run 
dorsad  in  the  five  branchial  arches  (Figs.  375  and  376)  and  join  the  paired 
descending  aorta.  The  first  and  second  pairs  of  aortic  arches  are  very 
small  and  originate  from  the  small  common  trunks  formed  by  the  bifurca- 
tion of  the  ventral  aorta  just  caudal  to  the  thyroid  gland.  The  fourth 
aortic  arch  is  the  largest.  From  the  apparent  fifth  arch  small  pulmonary 


Bulbils  cordis 


R.  ventrich'-§, 
Intervcnlriciilari^ 
foramen  % 


■'Foramen  ovale 

of  1.  atrium 

-Interatrial  foramen 
^Endocardial  cushions 


Fig.  372. — Dissection  of  the  heart  from  a 5.5  mm.  pig  embryo,  viewed  from  the  left  side.  X 14. 


arteries  are  developing.  There  is  evidence  that  this  pulmonary  arch  is 
really  the  sixth  in  the  series,  the  fifth  having  been  suppressed  in  develop- 
ment (cf.  Fig.  186  B).  Cranial  to  the  first  pair  of  aortic  arches,  the  de- 
scending aortal  are  continued  forward  into  the  maxillary  processes  as  the 
internal  carotids.  Caudal  to  the  aortic  arches,  the  descending  aortae 
converge,  unite  opposite  the  cardiac  end  of  the  stomach,  and  form  the 
median  dorsal  aorta  (Fig.  376).  Both  the  dorsal  and  descending  aortae 
give  off  paired,  dorsal  intersegmental  arteries.  From  the  seventh  pair  of 
these  arteries  (the  first  set  to  arise  from  the  medial  dorsal  aorta)  there  are 
developed  a pair  of  lateral  branches  to  the  upper  limb  buds.  These 
vessels  are  the  subclavian  arteries.  The  dorsal  aorta  also  sprouts  ventro- 
lateral arteries  to  the  glomeruli  of  the  mesonephros,  and  median  ventral 
arteries  to  the  gut  (Fig.  385).  Of  the  latter,  the  coeliac  artery  arises  oppo- 
site the  origin  of  the  hepatic  diverticulum.  The  vitelline  artery  takes 


THE  ANATOMY  OE  A SIX  MM.  PIG  EMBRYO 


363 


origin  by  two  or  three  trunks  caudal  to  the  dorsal  pancreas;  of  these,  the 
posterior  is  the  larger  and  persists  as  the  superior  mesenteric  artery. 

Opposite  the  lower  limb  buds,  the  dorsal  aorta  is  divided  for  a short 
distance.  From  each  division  there  arise,  laterad,  three  short  trunks  which 
unite  to  form  the  single  umbilical  artery  on  each  side.  The  middle  vessel 
is  the  largest  and  apparent^  becomes  the  common  iliac  artery.  A pair  of 
short  caudal  arteries,  much  smaller  in  size,  continue  the  descending  aortae 
into  the  tail  region. 


Spinal 
Ant.  cardinal 

Cervical  sinus 


Pericardial 

Atrial  junction  of  sinus 

Sinus  venosus 
Right  vitelline  vein 

Large  venous  sinusoid  of 

Hepatic  diverticulum  (cut) 

Yolk 

Portal  vein 

Cephalic  limb 
intestinal  loop 
Right  umbilical  vein 

Vitelline 

Caudal 
intestinal  loop 
Right  umbilical 


Dorsal 


Notochord 
nt.  cardinal 
vein 

Pharynx 

Pericardial 
cavity 

common 
cardinal  vein 
Left  horn  of  sinus  venosus 
Left  vitelline  vein 
Ductus  venosus 
Ant.  limb  bud 
nf.  vena  cava 
Dorsal  pancreas 
Left  vitelline  vein 

vitelline  vein 


Left  umbilical  vein 


Sup.  mesenteric  vein 
Left  umbilical  artery 
Post,  limb  bud 
Spinal  cord 


Fig.  373. — Ventral  reconstruction  of  a 6 mm.  pig  embryo,  showing  the  vitelline  and 
opened  umbilical  veins  (Vehe).  X 22.  In  the  small  orientation  figure  (cf.  Fig.  376)  the  vari- 
ous planes  are  indicated  by  broken  lines  — * *. 


The  Veins. — ’The  vitelline  veins,  originally  paired  throughout,  are  now 
represented  distally  by  a single  vessel,  which,  ramifying  in  the  wall  of  the 
yolk  sac,  enters  the  embryo  and  courses  cephalad  of  the  intestinal  loop 
(Fig.  373).  Crossing  to  the  left  side  of  the  intestine  and  ventral  to  it,  it 
is  joined  by  the  superior  mesenteric  vein  which  has  developed  in  the  mes- 
entery of  the  intestinal  loop.  The  trunk  formed  by  the  union  of  these  two 
vessels  becomes  the  portal  vein.  It  passes  along  the  left  side  of  the  gut  in 
the  mesentery,  and,  opposite  the  stem  of  the  dorsal  pancreas,  gives  off  a 


364 


THE  STUDY  OF  PIG  EMBRYOS 


small  branch,  a rudimentary  continuation  of  the  left  vitelline  vein,  which 
continues  cephalad  and  in  earlier  stages  connects  with  the  sinusoids  of  the 
liver.  The  portal  vein  then  bends  sharply  to  the  right,  dorsal  to  the  duo- 
denum, and,  in  the  course  of  the  right  vitelline  vein  (passing  between  the 
dorsal  and  ventral  pancreas  to  the  right  of  the  duodenum)  it  soon  enters 
the  liver  and  connects  with  the  liver  sinusoids.  The  portal  trunk  is  thus 
formed  by  persisting  portions  of  both  vitelline  veins,  and  receives  a new 
vessel,  the  superior  mesenteric  vein.  The  middle  portions  of  the  primitive 


Spinal  cord 
Anterior  cardinal  vein 
Cervical  sinus 

Pericardial  cavity 

R.  common  cardinal  vein 
Post,  cardinal  vein 
Eso phagus 

Large  venous  sinusoid  of  liver 
Anterior  limb  bud 
Inf.  vena  cava 
Post,  cardinal  vein 
Mesonephros  [cut  surface) 

R.  subcardinal  vein 

Venous  sinusoid  on  dorsum  of 
mesonephros 

Dorsal  aorta 
Notochord 


Notochord 


Pharynx 


T rachca 

L.  common 
cardinal  vein 
Lung 

Liver 

Stomach  {cut  edge) 

Omental  bursa 
Mesogastrium 

Mesonephros  (cut  surface) 

Capillary  anastomosis  between 
subcardinal  veins 
' — Vitelline  artery  in  dorsal 
mesentery 

Capillary  anastomosis  between 
subcardinal  veins 

'enoiis  sinusoid  on  dorsum  of 
mesonephros 


Spinal  cord 


Fig.  374. — Ventral  reconstruction  of  the  cardinal  and  subcardinal  veins  in  a 6 mm.  pig 
embryo,  showing  the  early  development  of  the  inferior  vena  cava  ( Vehe).  X 22.  In  the  small 
orientation  figure  (cf.  Fig.  376)  the  various  jdanes  are  indicated  by  broken  lines — * *. 


vitelline  veins  subdivide  into  the  network  of  liver  sinusoids.  Their 
proximal  vitelline  trunks  drain  the  blood  from  the  liver  and  open  into  the 
sinus  venosus  of  the  heart.  The  right  member  of  this  pair  is  much  the 
larger  (Fig.  371)  and  jiersists  as  the  jjroximal  portion  of  the  future  inferior 
vena  cava. 

The  umbilical  veins,  originating  in  the  walls  of  the  chorion  and  allantoic 
vesicle,  fuse  and  lie  caudal  and  lateral  to  the  allantoic  stalk  (cf.  Fig.  184). 
Before  the  stalk  enters  the  body,  they  separate  again  and  run  lateral  to  the 


THE  ANATOMY  OF  A SIX  MM.  PIG  EMBRYO 


365 

umbilical  arteries.  The  left  vein  is  much  the  larger.  Both,  after  receiving 
branches  from  the  posterior  limb  buds  and  from  the  body  wall,  pass 
cephalad' in  the  somatopleure  of  each  side  (Figs.  373  and  375).  Their 
course  is  first  cephalad,  then  dorsad,  until  they  enter  the  liver.  The  left 
vein  enters  a wide  channel,  the  ductus  venosiis,  which  carries  its  blood 


Spinal  cord 


Mesonephros 


Mclencephalon 


Int.  car- 
otid arterv 


Ant.  car- 
dinal vein 


Post,  cardinal  vein 


Mesonephric  arteries 


Mesencephalon 


Thyroid  Pk.P.2 

Myelencephalon 
Ph.  P.  3 


Ventral  aorta 

Ophthalmic 
vein 

Nasal  pit 
R.  atrium 

R.  ventricle 

L.  com.  cardinal  vein 
Liver 


R.  umbilical  vein 


Descending  aorta 


Ph.  P.  4 


Pul  monary  artery 
Lingiio-facial 
vein 
R.  com. 
cardinal  vein 


Sinus  venosus 


Intersegmenlal 

vein 


Diictus  venosus 


Com.  hepatic 
vein 

car- 
dinal vein 
. subclavian 


vein 

R.  subcar- 
dinal vein 


Pulmonary 

vein 


Fig.  375. — Reconstruction  of  a 7.8  mm.  pig  embryo,  showing  the  veins  and  aortic  arches  from 
the  left  side  (Thyng).  X 15.  Ph.P.  i,  2,  3,  4,  Pharyngeal  pouches. 


through  the  liver  and  thence  to  the  heart  by  way  of  the  right  vitelline 
trunk.  The  right  vein  joins  a large  sinusoidal  continuation  of  the  portal 
vein  in  the  liver.  This  common  trunk  drains  into  the  ductus  venosus. 

The  anterior  cardinal  veins  (Figs.  374  and  375)  are  formed  to  drain 
the  plexus  of  veins  on  each  side  of  the  head.  These  vessels  extend  caudad 


366 


THE  STUDY  OF  PIG  EMBRYOS 


and  lie  ventro-lateral  to  the  myelencephalon.  Each  receives  branches 
from  the  sides  of  the  myelencephalon,  then  curves  ventrad,  is  joined  by 
the  linguo-facial  vein  from  the  branchial  arches  and  at  once  unites  with  the 
posterior  cardinal  of  the  same  side  to  form  the  common  cardinal  vein. 
This,  as  already  explained,  opens  into  the  sinus  venosus. 

A posterior  cardinal  vein  develops  dorso-lateral  to  each  mesonephros 
(Figs.  374  and  375).  Running  cephalad,  they  join  the  anterior  cardinal 


Aortic  arch  i Seessel’s  pouch 


Aortic  arch  2 
Pharynx 
Thyroid 
Aortic  arch  t 


Ml 

Notochord 


Aortic  arch  4 

378 

Aortic  arch  6 and 
pulmonary  artery 
37> 

Esophagus 
38c 

Trachea 

381 

R.  lung 

382 


CasUac  artery 

383 

Ventral  pancreas 
Dorsal  pancreas 
Gall  bladder 

L.  umbilical  vein 
Vitelline 

386 

Cephalic  limb  of  intestinal 

Dorsal  aorta 


Int.  carotid  artery 
Ml 

Rathke’s  pouch 
Optic  recess 

Telencephalon 
Ventral  aorta 


Interventricular  foramen 
L.  horn  of  sinus  venosus 

^ ’’‘mhilical  vein 
Tail  gut 


383 

Spinal  cord 

384 

Metanepkric  anlage 

385 

L.  umbilical  artery 

nastomosis  between 
dorsal  aortcB 

illantoic  stalk 


L.  dorsal  aorta 


386  Mesonephric  duct 


rtery  to  mesonephros 
Caudal  limb  of  intestinal  loop 


Fig.  376. — Median  sagittal  reconstruction  of  a 6 mm.  pig  embryo  (Vehe).  X 16.5.  The 
numbered  heavy  lines  indicate  the  levels  of  the  transverse  sections  shown  in  Figs.  377-388. 
The  broken  lines  mark  the  outline  of  the  left  mesonephros  and  the  course  of  the  left  umbilical 
artery  and  vein ; the  latter  may  be  traced  from  the  umbilical  cord  to  the  liver  where  it  is  sectioned 
longitudinally. 


veins.  When  the  mesonephroi  become  prominent,  as  at  this  stage,  the 
middle  third  of  each  posterior  cardinal  is  broken  up  into  sinusoids.  Sinus- 
oids extend  from  the  posterior  cardinal  vein  ventrally  around  both  the 
lateral  and  medial  surfaces  of  the  mesonephros.  The  median  sinusoids 
anastomose  longitudinally  and  form  the  subcardinal  veins,  right  and  left. 


THE  ANATOMY  OF  A SIX  MM.  PIG  EMBRYO 


367 


The  subcardinals  lie  along  the  median  surfaces  of  the  mesonephroi,  more 
ventrad  than  the  posterior  cardinals  with  which  they  are  connected  at 
either  end.  There  is  a transverse  capillary  anastomosis  between  the  two 
subcardinals,  cranial  and  caudal  to  the  permanent  trunk  of  the  vitelline 
artery  (Fig.  374).  The  right  vessel  is  connected  with  the  liver  sinusoids 
through  a small  vein  (which  develops  in  the  mesenchyme  of  the  caval 
mesentery)  located  to  the  right  of  the  mesogastrium  (Fig.  383).  This  vein 
now  carries  blood  direct  to  the  heart  from  the  right  posterior  cardinal  and 
right  subcardinal,  by  way  of  the  liver  sinusoids  and  the  right  vitelline  trunk 
(common  hepatic  vein).  Eventually,  the  unpaired  inferior  vena  cava  forms 
in  the  course  of  these  four  vessels. 

Transverse  Sections  of  a Six  Mm.  Pig  Embryo 

Having  acquainted  himself  with  the  anatomy  of  the  embryo  from  the 
study  of  dissections  and  reconstructions,  the  student  is  prepared  to 
examine  serial  sections.  To  interpret  the  structures  encountered,  there 
must  be  constant  references  to  the  figures  already  described  which  show 
the  positions  of  the  organs.  Determine  the  exact  plane  of  a section  with 
reference  to  these  illustrations,  and  especially  Fig.  376.  Representative 
levels  are  described  on  the  following  pages;  the  position  of  each  is  indicated 
by  the  heavy,  numbered  lines  on  Fig.  376.  These  sections  are  drawn  from 
the  cephalic  surface,  so  that  the  right  side  of  the  embryo  is  at  the  reader’s 
left. 

Sections  through  the  Cephalic  Flexure. — The  earliest  sections  cut  the  mesencephalon, 
metencephalon,  and  thin-roofed  myelencephalon.  At  levels  which  include  two  separate 
portions  of  the  brain,  the  smaller  is  the  diencephalon,  the  larger  a longitudinal  section  of  the 
myelencephalon.  In  the  intermediate  mass  of  mesenchyme  lie  the  internal  carotid  arteries 
(cf.  Fig.  377).  Lateral  to  them  are  numerous  branches  of  the  anterior  cardinal  veins. 
Midway  along  the  sides  of  the  hind-brain  are  the  apices  of  the  otocysts. 

Section  through  the  Myelencephalon  and  Otocysts  (Fig.  377). — As  the  head  is  bent 
nearly  at  right  angles  to  the  body,  the  brain  is  cut  twice ; this  section  passes  lengthwise 
through  the  myelencephalon  and  transversely  through  the  diencephalon.  The  cellular  walls 
of  the  myelencephalon  show  a series  of  six  paired  constrictions,  the  neuromeres.  Lateral 
to  the  fourth  pair  of  neuromeres  are  the  otocysts,  which  show  a median  outpocketing  at  the 
point  of  entrance  of  the  endolymph  duct.  The  ganglia  of  the  nn.  trigeminus,  facialis, 
acusticus,  and  the  superior  ganglion  of  the  glossopharyngeal  nerve  occur  in  order  on  each 
side.  Sections  of  the  anterior  cardinal  vein  appear  in  several  places,  and  ventral  to  the 
diencephalon  are  the  internal  carotid  arteries. 

Passing  along  down  the  series  into  the  pharynx  region,  observe  the  first,  second,  and 
third  pharyngeal  pouches.  Their  dorsal  diverticula  come  into  contact  with  the  ectoderm 
, of  the  branchial  grooves  and  form  the  closing  plates. 

Section  through  the  Branchial  Arches  and  the  Eyes  (Fig.  378). — The  section  passes 
lengthwise  through  the  four  branchial  arches,  the  fourth  sunken  in  the  cervical  sinus. 
The  mandibular  processes  of  the  first  arch  have  united  to  form  the  mandible,  or  lower  jaw; 
i the  maxillary  processes  of  the  future  upper  jaw  lie  across  the  stomodeal  space  in  the  separate 


368 


THE  STUDY  OF  PIG  EMBRYOS 


Fourth 


Myelcncephalon 


Int.  carotid  art 


A lit.  cardinal  vein 


Ant.  cardinal  vein 


Fig.  377. — Transverse  section  through  the  myelcncephalon  and  otocysts  of  a 6 mm,  pig 
embryo.  X .26.5.  Near.  1-6,  neuromeres. 


M andiUe 


Fig.  378. — Transverse  section  through  the  branchial  arches  and  eyes  of  a 6 mm.  pig  embryo. 

X 26.5.  .Y,  aortic  arch  4. 


Near. 

Gang.juglare  n. 

Neur.  5 
Otocyst 
Neur.  4 
Neur. 
Neur. 

Neur. 


Spinal  cord 


M yotome 


Descending  aorta 
Branchial  arch  4 
Branchial  arch  j 

Branchial  arch  2 


Lens  of 


vesicle 


. super  ills  n.  g 


cardinal  vein 

■ acusticum  n.  8 

■ geniculi  n.  7 


Ant.  cardinal 


Pharyngeal  pouch 
Aortic  arch 


Pharyngeal  pouch 


.semiliinare  n.  5 


THE  ANATOMY  OF  A SIX  MM.  PIG  EMBRYO 


369 


section  of  the  bent  head.  Dorsad  is  the  spinal  cord,  with  the  first  pair  of  cervical  ganglia, 
and  laterad  the  first  cervical  myotonies.  The  pharynx  is  cut  across  between  the  third  and 
fourth  branchial  pouches.  In  its  floor  is  a prominence,  the  anlage  of  the  epiglottis.  Ventral 
to  the  pharynx,  the  ventral  aorta  gives  off  two  pairs  of  vessels.  The  larger  pair  are  the 
fourth  aortic  arches  w'hich  curve  dorsad  around  the  pharynx  to  enter  the  descending  aortce. 
The  smaller  third  aortic  arches  enter  the  third  branchial  arches  on  each  side.  A few  sections 
higher  up  in  the  series,  the  ventral  aorta  bifurcates,  and  the  right  and  left  trunks  thus  formed 
give  off  the  first  and  second  pair  of  aortic  arches.  Cranially,  in  the  angle  between  their 
common  trunks,  lies  the  median  thyroid  anlage.  The  anterior  cardinal  veins  are  located 
dorso-lateral  to  the  descending  aortae.  The  end  of  the  head  is  cut  through  the  dience- 
phalon and  the  optic  vesicles.  On  the  left  side  of  the  figure,  the  lens  vesicle  may  be  seen  still 
connected  with  the  ectoderm.  The  corresponding  optic  cup  appears  asymmetrical  because 
it  is  cut  through  the  chorioid  fissure.  The  cup  is  differentiated  into  a thick  inner,  and  a thin 
outer  layer ; these  form  the  nervous  and  pigment  layers  of  the  retina  respectively. 

Section  through  the  Tracheal  Groove,  Bulbus  Cordis  and  Olfactory  Pits  (Fig.  379). — 
The  ventral  portion  of  the  figure  shows  a section  through  the  tip  of  the  head.  The  telen- 


Myotome 


aorta 


Pericardial  cavity 


Bulbus  cordis 


Telencephalon 


Spinal  cord 


Ant.  cardinal 


Olfactory 


Fig.  379. — Transverse  section  through  the  bulbus  cordis  and  olfactory  pits  of  a 6 mm.  pig 

embryo.  X 26.5. 


cephalon  is  not  prominent.  The  ectoderm  is  thickened  and  sUghtly  invaginated  ventro- 
laterad  to  form  the  anlages  of  the  olfactory  pits.  These  deepen  in  later  stages  and  become 
the  nasal  cavities.  In  the  dorsal  portion  of  the  section  may  be  seen  the  cervical  spinal  cord, 
the  notochord  just  ventral  to  it,  the  descending  aortce,  and  ventro-lateral  to  them  the  anterior 
cardinal  veins.  The  naso-pharynx  now  is  small  with  a vertical  groove  in  its  floor.  This 
is  the  tracheal  groove  and  more  caudad  it  will  become  the  cavity  of  the  trachea.  The  bulbus 
24 


370 


THE  STUDY  OF  PIG  EIVIBRYOS 


cordis  lies  in  the  large  pericardial  cavity.  On  either  side  the  section  cuts  through  the 
cephalic  portions  of  the  atria.  These  will  become  larger  farther  caudad  in  the  series. 

Section  through  the  Heart  (Fig.  380). — The  heart  lies  in  the  pericardial  cavity.  Both 
the  atrium  and  ventricle  are  divided  incompletely  into  two  chambers.  A partial  inter- 
ventricular septum  leaves  the  ventricles  in  communication  dorsad.  The  septum  primum  is 
complete  in  this  section,  but  cephalad  in  the  series  there  is  an  interatrial  foramen  (cf.  Fig. 
372).  'T\\q  foramen  ovale  has  not  formed  in  this  particular  embryo.  The  myocardium  of 

the  ventricles  is  a spongy  layer,  much  thicker  than  that  of  the  atrial  wall.  Lateral  to  the 
descending  aortae  are  the  common  cardinal  veins.  The  right  common  cardinal  opens  into 
the  sinus  venosus  which  in  turn  empties  into  the  right  atrium,  its  opening  being  guarded 
by  the  two  valves  of  the  sinus  venosus.  The  entrance  of  the  left  common  cardinal  into  the 
sinus  venosus  is  somewhat  more  caudad  in  the  series.  The  trachea  has  now  separated 
from  the  esophagus  and  lies  ventral  to  it.  Both  trachea  and  esophagus  are  surrounded 
by  a condensation  of  mesenchyme  which  will  transform  into  the  muscular  and  fibrous  coats. 


■Septum 


Spinal  cord 


Myotome 


Descending  aorta 
Esophagus 

Sinus  venosus' 
Valve  of  sinus  venosus' 

R.  alriunh 
Atrio-ventricular  foramen 

Interventricular  septum 

R.  ventricle 
Somatoplcurc 


L.  common  cardinal  vein 


Trachea 


L.  atrium 


L.  ventricl 


T nterventricular  foramen 
Pericardial  cavity 


Fig.  380. — Transverse  section  through  the  four  chambers  of  the  heart  of  a 6 mm.  pig  embryo. 

X 26.5. 


Section  through  the  Lung  Buds  and  Septum  Transversum  (Fig.  381). — The  section 
passes  through  the  bases  of  the  upper  limh  buds.  A pair  of  spinal  nerves,  with  ganglia 
and  roots,  extends  from  the  spinal  cord  into  these  anlages.  The  tips  of  the  ventricles,  lying 
in  the  pericardial  cavity,  still  show  in  this  section.  Dorsally,  the  pericardial  cavity  has  given 
place  to  the  pleuro-pcritoiieal  cavity.  Projecting  ventrad  into  this  cavity  are  the  cranial 
ends  of  the  mesonephric  folds  in  which  the  posterior  cardinal  veins  partly  lie.  Into  the  floor 
of  the  pleuro-peritoneal  cavities  bulge  the  dorsal  lobes  of  the  liver,  embedded  in  splanchnic 
mesenchyme.  This  mesenchyme  is  continuous  with  that  of  the  somatopleure,  and  forms 
a transverse  partition  between  the  liver  and  heart,  complete  ventrally.  This  is  the  septum 
transversum  which  takes  part  in  forming  the  ligaments  of  the  liver  and  is  the  chief 
anlage  of  the  diaphragm.  The  two  proximal  trunks  of  the  vitelline  veins  pass  through  the 


THE  ANATOMY  OF  A SIX  MM.  PIG  EMBRYO 


371 


Septum 


Pericardial  cavi 


Spinal  cord 


U ppcr  limb  bud 

Post,  cardinal  vein 

Esophagus 
Dorsal  lobe  of  liver 

Omental  bursa 

L.  vitelline  vein 


L.  ventricle 


Descending 


Pieuro-peritoneal 
R.  lung 
R.  vitelline 


Fig.  381. — Transverse  section  through  the  right  lung  bud  and  septum  transversum  of  a 6 mm. 

pig  embryo.  X 26.5. 


Spinal 

Notochord 
Dorsal 

Peritoneal  cav 

Omental 

Common  hepatic  vein 
(R.  vitelline) 

R.  ventral  lobe 


L.  ventricle 


Fig.  382. — Transverse  section  through  the  stomach  of  a 6 mm.  pig  embryo.  X 26.5. 


cord 


Spinal  nerve 


Post,  cardinal  vein 
U pper  limb  bud 
Stomach 


L.  ventral  lobe  of  liver 


372 


THE  STUDY  OF  PIG  EMBRYOS 


septum.  Projecting  laterally  into  the  pleuro-peritoneal  cavities  are  ridges  of  mesenchyme- 
covered  by  splanchnic  mesoderm  in  which  the  lungs  develop  as  lateral  buds  from  the  caudal 
end  of  the  trachea.  The  right  lung  bud  is  shown  in  the  figure.  Between  the  esophagus 
and  the  lung  is  a crescent-shaped  cavity,  the  cranial  end  of  the  omental  bursa. 

Section  through  the  Stomach  (Fig.  382). — The  section  passes  through  the  upper 
limb  buds  and  just  caudal  to  the  point  at  which  the  descending  aortas  unite  to  form  the 
median  dorsal  aorta.  As  the  liver  develops  in  early  stages,  it  comes  into  relation  with  the 
caval  mesentery,  along  the  dorsal  body  wall,  at  the  right  side  of  the  dorsal  mesogastrium. 
The  space  between  the  liver  and  plica  to  the  right,  and  the  stomach  and  its  omenta  to  the 
left,  is  a caudal  continuation  of  the  omental  bursa.  The  dorsal  wall  of  the  stomach  is  rotated 
to  the  embryo’s  left,  its  ventral  wall  to  the  right.  The  liver  shows  a pair  of  dorsal  lobes, 
and  contains  large  blood  spaces  and  networks  of  sinusoids  lined  with  endothelium.  Ventral 
to  the  liver,  the  tips  of  the  ventricles  still  appear. 

Section  through  the  Hepatic  Diverticulum  (Fig.  383). — The  upper  limb  buds  are 
prominent  in  this  section.  The  mesonephric  folds  show  the  tubules  and  glomeruli  of  the 
mesonephroi,  and  the  posterior  cardinal  veins  are  connected  with  the  mesonephric  sinu- 
soids. From  the  dorsal  attachment  of  the  liver  a ridge  continues  down  into  this  section 


Fig.  383. — Transverse  section  through  the  hepatic  diverticulum  of  a 6 mm.  pig  embryo. 

X 26.5. 


Myotome 

Post,  cardinal  vein 
U p per  limb  bud 


Spinal  cord 


Notochord 
Post,  cardinal  vein 
Dorsal  aorta 


Inf.  vena  cava 

Portal  vein 
R.  umbilical  vein 
Hepatic  diverticulum 


Pericardial  cavity 


Mesoduodenum 
Dorsal  lobe  of  liver 


L.  vitelline  vein 


L.  umbilical  vein 


it  lies  on  the  dorsal  body  wall  just  to  the  right  (left  in  figure)  of  the  mesentery.  In  this 
ridge  is  a small  vein  which  connects  cranially  with  the  liver  sinusoids,  caudally  with  the 
right  subcardinal  vein.  As  it  later  forms  a portion  of  the  inferior  vena  cava,  the  ridge  in 
which  it  lies  is  termed  the  caval  mesentery.  The  right  dorsal  lobe  of  the  liver  contains  a 
large  blood  space  into  which  the  portal  vein  opens.  The  duodenum  is  ventral  to  the  position 
occupied  by  the  stomach  in  the  previous  section.  There  is  given  off  from  it,  ventrad  and 
to  the  right,  the  hepatic  diverticulum.  In  sections  cephalad,  small  ducts  from  the  liver 
trabeculae  may  be  traced  into  connection  with  it.  In  the  left  ventral  lobe  of  the  liver,  a 
large  blood  space  indicates  the  position  of  the  left  umbilical  vein  on  its  way  to  the  ductus 
venosus.  The  mesothelial  investment  of  the  liver  is  the  tissue  of  the  expanded  ventral 
mesentery  into  which  it  grew.  Between  the  stomach  and  liver,  the  ventral  mesentery 


THE  ANATOMY  OF  A SIX  MM.  PIG  EMBRYO 


373 


is  called  the  lesser  omentum;  between  the  liver  and  ventral  body  wall,  the  falciform 
ligament. 

Section  through  the  Pancreatic  Anlages  (Fig.  3S4). — At  this  level,  the  upper  limb  buds 
still  show.  The  mesonephroi  are  prominent  and  marked  by  their  large  Bowman’s  capsules 
and  glomeruli,  located  mesad.  The  right  posterior  cardinal  vein  is  broken  up  into  mesone- 
phric sinusoids.  The  vein  in  the  caval  mesentary  will  connect  with  the  right  subcardinal 
vein  a few  sections  lower.  The  anlage  of  the  dorsal  pancreas  is  seen  extending  from  the 
duodenum  dorsad  into  the  mesenchyme  of  the  mesentery.  It  soon  bifurcates  into  a dorsal 
and  right  lobe,  of  which  the  latter  is  slightly  lobulated.  Ventro-lateral  to  the  duodenum, 
the  anlage  of  the  ventral  pancreas  is  cut  across;  it  may  be  traced  cephalad  in  the  series  to 
its  origin  from  the  hepatic  diverticulum.  To  the  right  of  the  ventral  pancreas  lies  the 
portal  vein  (at  this  level  the  portion  contributed  by  the  right  vitelline).  To  the  left  of  the 
dorsal  pancreas  is  seen  the  remains  of  the  left  vitelline  vein.  The  ventral  lobes  of  the  liver 
are  just  disappearing  at  this  level.  In  the  mesenchyme  that  connects  the  liver  with  the 
ventral  body  wall  lie  on  each  side  the  umbilical  veins,  the  left  being  the  larger.  Between 
the  veins  is  the  extremity  of  the  hepatic  diverticulum  which  becomes  the  gall  bladder. 
The  body  wall  is  continued  ventrad  to  form  a short  umbilical  cord. 


Dorsal 

R.  post,  cardinal  vein 
Glomerulus  of 

Inf.  vena  cava 


Portal  vein 
R.  umbilical  vein 

Gall  bladder 


Spinal  cord 


L.  post,  cardinal  vein 
Mesonephros 
U p per  limb  bud 


Mesentery 
Dorsal  pancreas 
L.  vitelline  vein 
Duodenum 
L.  umbilical  vein 


Ventral  pancreas 

Fig.  384. — Transverse  section  through  the  pancreatic  anlages  of  a 6 mm.  pig  embryo.  X 26.5. 


Section  through  the  Intestinal  Loop  and  Lower  Limb  Buds  (Fig.  385). — As  the  posterior 
half  of  the  embryo  is  curved  in  the  form  of  a half  circle,  sections  caudal  to  the  liver,  like 
this  one,  pass  also  through  the  lower  end  of  the  body.  Two  sections  of  the  embryo 
are  thus  seen  in  one,  their  ventral  aspects  facing  each  other  and  connected  by  the  lateral 
body  wall.  In  the  dorsal  part  of  the  section  the  mesonephroi  are  prominent,  with 
large  posterior  cardinal  veins  lying  dorsal  to  them  and  mesonephric  arteries  branching 
laterally  from  the  aorta  and  passing  to  the  glomeruli.  The  trunk  of  the  vitelline  artery  is  the 
delicate  tube  taking  origin  ventrally  from  the  aorta.  It  may  be  traced  into  the  mesentery, 
and  through  it  onto  the  wall  of  the  yolk  sac.  On  either  side  of  the  vitelline  artery  are  the 
subcardinal  veins,  the  right  being  the  larger.  In  the  mesentery  may  be  seen  two  sections 
of  the  intestinal  loop  (the  small  intestine  being  cut  lengthwise,  the  large  intestine 
transversely),  and  also  sections  of  the  vitelline  artery  and  veins.  In  the  lateral  body  walls, 


374 


THE  STUDY  OF  PIG  EMBRYOS 


ventral  to  the  mesonephros,  occur  the  umbilical  veins.  The  left  vein  is  large  and  cut 
lengthwise'  the  right  vein  is  cut  obliquely  twice. 

In  the  ventral  portion  of  the  section,  the  lower  limb  buds  are  prominent  laterally.  A 
pair  of  large  arteries,  the  common  iliacs,  branch  from  the  aorta  and  may  be  traced  into  con- 
nection with  the  umbilical  arteries,  d'he  colon,  supported  by  a short  mesocolon,  lies 
in  the  cadom  near  the  midplane.  On  each  side  are  the  caudal  ends  of  the  mesonephric 
folds,  here  small  and  each  showing  a section  of  the  mesonephric  duct  and  a single  vesicular 


Fig.  385. — Transverse  section  through  the  intestinal  loop  and  lower  limb  buds  of  a 6 mm. 

pig  embryo.  X 26.5. 

anlage  of  the  mesonephric  tubules.  The  mesonephric  ducts  are  cut  as  they  curve  around 
from  their  position  in  the  dorsal  portion  of  the  section.  The  tip  of  the  recurved  tail  shows. 

Section  through  the  Primitive  Segments  and  Spinal  Cord  (Fig.  386). — In  the  interval 
since  the  level  last  described  are  encountered  sections  which  show  symmetrical  longitu- 
dinal views  of  the  body  wall,  mesonephroi,  mesentery,  aorta,  and  notochord.  This  section 
is  near  the  end  of  the  series,  and,  as  the  body  is  here  curved,  it  is  really  a frontal  section. 
At  the  left  side  of  the  spinal  cord,  the  oval  cellular  masses  are  the  spinal  ganglia  cut  across. 


THE  ANATOMY  OF  A SEX  MM.  PIG  EMBRYO 


375 


The  ectoderm,  arching  over  the  segments,  indicates  their  position.  Each  segment  shows 
an  outer  dense  layer,  the  dermatome,  lying  just  beneath  the  ectoderm.  This  plate  curves 
lateral  to  the  spindle-shaped  myotome,  which  gives  rise  to  the  voluntary  muscle.  Next 
comes  a diffuse  mass  of  mesenchyme,  the  sclerotome,  which,  with  its  fellow  of  the  opposite 
side,  eventually,  surrounds  the  spinal  cord  and  forms  the  anlage  of  a vertebra.  A pair  of 
spinal  nerves  and  spinal  ganglia  are  developed  opposite  each  somite,  and  pairs  of  small 
vessels  are  seen  between  the  segments.  These  are  dorsal  intersegmental  arteries. 

Section  through  the  Cloaca  and  Metanephric  Anlages 
(Fig  387). — Having  now  studied  sections  at  various  levels  to 
near  the  end  of  the  series,  the  next  step  is  to  examine  sections 
through  the  caudal  region  and  study  the  anlages  of  the 
urogenital  organs.  Owing  to  the  curvature  of  the  embryo, 


Spinal  ganglion 
Intersegmental 
artery 
Myotome' 

Dermal  une 

Sclerotome 
Ectoderm 
Spinal  cord 


Fig.  386. — Transverse  sec- 
tion through  the  primitive 
segments  and  spinal  cord  of  a 
6 mm.  pig  embryo.  X 45. 


R.  umbilical  vein 
R.  umbilical  artery 

Tail 


Mesonephric  duct 
Metanephric  anlage 


Spinal  cord 


Ventral  body  wall 

L.  umbilical  artery 
~L.  umbilical  vein 
Allantoic  stalk 


Cloaca 

Rectum 

Notochord 


through 


Fig.  387. — Transverse  section 

metanephroi  of  a 6 mm.  pig  embryo.  X 45 


cloaca  and 


Fig.  388. — Transverse  section  through  the  umbilical  vessels,  allantois  and  cloaca  of  a 6 mm 

pig  embryo.  X 45- 


it  is  necessary  to  go  cephalad  in  the  series.  The  metanephroi  appear  as  dorsal 
evaginations  from  the  mesonephric  ducts,  just  before  their  entrance  into  the  cloaca. 


376 


THE  STUDY  OF  PIG  EMBRYOS 


Each  consists  of  an  epithelial  layer  surrounded  by  a condensation  of  mesenchyme.  Traced 
a few  sections  cephalad,  the  mesonephric  ducts  open  into  the  lateral  diverticula  of  the 
cloaca,  which,  irregular  in  outline  because  it  is  sectioned  obliquely,  lies  ventral  to  them 
and  receives  dorsad  the  rectum.  Caudal  to  the  cloaca,  in  this  embryo,  the  tail  bends 
abruptly  cephalad  and  to  the  right.  The  blind  prolongation  of  the  hindgut  may  be  traced 
out  into  this  portion  of  the  tail  until  it  ends  in  a sac-like  dilatation. 

Section  through  the  Umbilical  Vessels,  Allantois  and  Cloaca  (Fig.  388). — The  present 
section  passes  through  the  bases  of  the  limb  buds  at  the  level  where  the  allantoic  stalk, 
curving  inward  from  the  umbilical  cord,  opens  into  the  cloaca.  At  either  side  of  the  allan- 
toic stalk  may  be  seen  oblique  sections  of  the  umbilical  arteries,  and  lateral  to  these  the 
large  left  and  small  right  ■umbilical  vein.  The  mesonephric  ducts  occupy  the  mesonephric 
ridges  which  project  into  small  caudal  prolongations  of  the  coelom.  Midway  between  the 
ducts  lies  the  rectum,  dorsal  to  the  cloaca.  The  tip  of  the  tail  is  seen  in  section  at  the 
left  of  the  figure. 

(B)  THE  ANATOMY  OF  TEN  TO  TWELVE  MM.  PIG  EMBRYOS 

This  is  the  most  instructive  single  stage  of  development.  The  anlages 
and  relations  of  nearly  all  the  important  organs  are  represented,  and  yet 
the  embryo  is  not  so  complex  as  to  confuse  a beginner.  Embryos  between 
8 and  14  mm.  long  may  be  used  equally  well  in  conjunction  with  the 
appended  descriptions.  A human  embryo  of  12  mm.  is  shown  in  Fig.  64; 
its  internal  anatomy  is  fundamentally  the  same. 

External  Form. — The  head  is  relatively  large  on  account  of  the 
increased  size  of  the  brain  (Fig.  389).  The  third  branchial  arch  is  still 
visible  in  the  embryo,  but  the  fourth  arch  has  sunken  in  the  cervical  sinus; 
usually,  both  disappear  at  a slightly  later  stage.  The  olfactory  pits  form 
elongated  grooves  on  the  under  surface  of  the  head,  and  the  lens  of  the  eye 
lies  beneath  the  ectoderm,  surrounded  by  the  optic  cup.  The  maxillary 
and  mandibular  processes  of  the  first  branchial  arch  are  large;  the  former 
show  signs  of  fusing  with  the  median  nasal  processes  to  form  the  upper  jaw, 
while  the  mandibular  processes  have  united  already.  Small  tubercles,  the 
anlages  of  the  external  ear,  bound  the  first  branchial  groove,  which  itself 
becomes  the  external  acoustic  meatus. 

At  the  cervical  bend,  the  head  is  flexed  at  right  angles  with  the  body, 
bringing  the  ventral  surface  of  the  head  close  to  the  trunk,  and  it  is 
probably  owing  to  this  flexure  that  the  third  and  fourth  branchial  arches 
buckle  inward  to  form  the  cervical  sinus.  Dorsad,  the  trunk  forms 
a long  curve,  more  marked  opposite  the  posterior  extremities.  The 
reduction  in  the  trunk  flexures  results  from  the  increased  size  of  the  heart, 
liver,  and  mesonephroi.  These  organs  are  indicated  through  the  translu- 
cent body  wall,  and  the  position  of  the  septum  transversum  may  be  noted 
between  the  heart  and  the  liver  (cf.  Fig.  390).  The  limb  buds  are  larger 
and  the  umbilical  cord  is  prominent  ventrad.  Dorsally,  the  mesodermal 
segments  occur,  and,  extending  in  a curve  between  the  bases  of  the  limb 
buds,  is  the  milk  line,  a thickened  ridge  of  ectoderm  which  forms  the 


THE  ANATOMY  OF  TEN  TO  TWELY'E  MM.  PIG  EMBRYOS 


377 


Myelencephalon 


Mesodermal  segment 


Umbilical  cord 


Lower  limb  bud 
Fig.  389. — Pig  embryo  of  10  mm.  X 7. 


Cervical  sinus 
Upper  limb  hud 


Maxillary  process 
Ma7idihiilar  process 
Olfactory  pit 


Milk  Ime 


Branchial  groove  1 
Hyoid  arch 


Cervical  flexure 
Branchial  arch  3 


Yolk  sac 


Gang.  n.  5 
Gang.  nn.  7 and  8 

N.  facialis 
Gang,  superius  n.  g 
Gang,  jiigiilare  n.  10 
Gang,  petrosum  n.  9 
Gang.  Froriep 
Gang,  nodosum  n.  10 

y.  accessorius 
y.  hypoglossus 

Atrium 
Lung 
Gang.  cerv.  8 

Septum  transversiim 


Metencephalon  N.  trochlear  is 


Mesencephalon 

y.  oculomotorius 


'encephalon 

ic  r.  n.  5 
y.  opticus 

Maxillary  r.  n.  5 
Telencephalon 

Mandibular  r.  n.  5 
Chorda  tympani  n.  7 


Ventricle 


Mesonephros 


Fig.  390. — Lateral  dissection  of  a 10  mm.  pig  embryo.  X 10.5. 


Gang,  thorac.  10 


Umbilical  cord 
Genital  tubercle 


378 


THE  STUDY  OF  PIG  EMBRYOS 


anlages  of  the  mammary  glands.  The  tail  is  long  and  tapering.  Between 
its  base  and  umihlical  cord  is  the  genital  tubercle  (Fig.  390). 

Nervous  System  and  Sense  Organs. — The  Brain. — Five  distinct  regions 
may  be  distinguished  (Figs.  390  to  392):  (i)  The  telencephalon,  with  its 
rounded  lateral  outgrowths,  the  cerebral  hemispheres.  Their  cavities,  the 
atcral  ventricles,  communicate  by  interventricular  foramina  with  the  third 
ventricle.  (2)  The  dience phaion  shows  a laterally  flattened  cavity,  the  third 
ventricle.  Ventro-laterally  from  the  diencephalon  pass  off  the  optic  stalks, 
and.  an  evagination  of  the  midventral  wall  is  the  anlage  of  the  posterior 


Accessory  gang,  i 
Accessory  gang.  2 


1 .-f . gang.  3 


vT, — M yelencephalon 


Acc.  gang.  4 


Ga)ig.  Froriep 
N.  II 
Cerv.  gang,  i 


Cerv.  gang.  2 


. snperius 
Gang,  jugulare 


Gang,  petrosmn 
N.  9 
N.  II 


Gang,  nodosum 
N.  12 


Fig.  391. — Dissection  of  the  postotic  cranial  nerves  and  ganglia  of  a 15  mm.  pig  embryo, 

viewed  from  the  right  side,  X 25. 


hypophyseal  lobe.  (3)  The  mesencephalon  is  undivided,  but  its  cavity 
becomes  the  cerebral  aqueduct  leading  caudally  into  the  fourth  ventricle. 
(4)  The  metencephalon  is  separated  from  the  mesencephalon  by  a con- 
striction, the  isthmus.  Dorso-laterally  it  becomes  the  cerebellum,  ventrally 
the  pons.  (5)  The  elongated  myelencephalon  is  roofed  over  by  a thin, 
non-nervous  ependymal  layer.  Its  ventro-lateral  wall  is  thickened  and 
still  gives  internal  indication  of  the  neuromeres.  The  cavity  of  the 
metencephalon  and  myelencephalon  is  the  fourth  ventricle. 

The  Cranial  Nerves. — Of  the  twelve  cranial  nerves,  all  but  the  first 
(olfactory)  and  sixth  (abducens)  are  represented  in  Fig.  390;  (2)  The  optic 
nerve  is  represented  by  the  optic  stalk,  cut  through  in  this  illustration. 
(3)  The  oculomotor,  a motor  nerve  to  four  of  the  eye  muscles,  takes  origin 


THE  ANATOMY  OF  TEN  TO  TWELVE  MM.  PIG  EMBRYOS 


379 


from  the  ventro-lateral  wall  of  the  mesencephalon.  (4)  The  trochlear  nerve 
fibers,  motor,  to  the  superior  oblique  muscle  of  the  eye,  arise  from  the 
ventral  wall  of  the  mesencephalon,  turn  dorsad  and  cross  at  the  isthmus, 
thus  emerging  on  the  opposite  side.  From  the  myelencephalon  appear 
in  order:  (5)  the  n.  trigeminus,  mixed,  with  its  semilunar  ganglion  and  three 
branches,  the  ophthalmic,  maxillary,  and  mandibtilar;  (6)  the  n.  abdiicens, 
motor,  from  the  ventral  wmll  to  the  external  rectus  muscle  of  the  eye;  (7) 
the  n.  facialis,  mixed,  with  its  geniculate  ganglion  and  its  chorda  tympani, 
facial,  and  superficial  petrosal  branches  in  the  order  named;  (8)  the  n. 
acusticus,  sensory,  arising  cranial  to  the  otocyst,  with  its  acoustic  ganglion 
and  sensory  fibers  to  the  internal  ear;  (9)  caudal  to  the  otocyst  the  n. 
glossopharyngeus,  mixed,  with  its  superior  and  petrosal  ganglia;  (10)  the 
n.  vagus,  sensory,  with  its  jugular  and  nodose  ganglia;  ( 1 1)  the  n.  accessorius, 
whose  motor  fibers  take  origin  from  the  lateral  wall  of  the  spinal  cord  and 
myelencephalon  between  the  jugular  and  sixth  cervical  ganglia;  the 
internal  branch  of  the  n.  accessorius  accompanies  the  vagus;  the  external 
branch  leaves  it  between  the  jugular  and  nodose  ganglia  and  supplies  the 
sterno-mastoid  and  trapezius  muscles;  (12)  the  n.  hypoglossus,  motor, 
arising  by  five  or  six  fascicles  from  the  ventral  wall  of  the  myelencephalon ; 
its  trunk  passes  lateral  to  the  nodose  ganglion  and  supplies  the  muscles 
of  the  tongue. 

The  orderly  innervation  of  the  four  branchial  arches  by  the  fifth, 
seventh,  ninth,  and  tenth  nerves  is  not  so  diagrammatic  as  in  the  6 mm. 
embryo  but  these  relations  continue  nevertheless. 

A nodular  chain  of  ganglion  cells  extends  caudad  from  the  jugular  ganglion  of  the 
vagus  (Fig.  391).  These  have  been  interpreted  as  accessory  vagus  ganglia.  They  may, 
however,  be  continuous  with  Froriep’s  ganglion  which  sends  sensory  fibers  to  the  n.  hypo- 
glossus. In  pig  embryos  of  1 5 mm.  this  chain  is  frequently  divided  into  four  or  five  gang- 
lionic masses,  of  which  occasionally  two  or  three  (including  Froriep’s  ganglion)  send 
fibers  to  the  root  fascicles  of  the  hypoglossal  nerve  (Fig.  391). 

The  Spinal  Nerves. — Each  of  these  has  its  own  spinal  ganglion,  from 
which  the  dorsal  root  fibers  are  developed  (Figs.  390  and  406).  The 
motor  fibers  take  origin  from  the  ventral  cells  of  the  neural  tube  and  form 
the  ventral  roots  which  join  the  dorsal  roots  in  the  nerve  trunk. 

The  Sense  Organs.' — -The  olfactory  pits  are  deep  foss®,  flanked  by  the 
nasal  processes.  The  stalked  optic  cup  is  prominent  and  the  lens  vesicle 
detached.  The  otocyst  is  a compressed  oval  vesicle  with  a tubular  endo- 
lymph  duct  growing  dorsad  from  its  median  side. 

Digestive  and  Respiratory  Systems. — Pharynx. — Dorsally,  the  ante- 
rior lobe  of  the  hypophysis  is  long  and  forks  at  its  end  (Figs.  392  and  393). 
In  the  floor  of  the  pharynx  are  the  anlages  of  the  tongue  and  epiglottis 
(Fig.  84  B).  From  the  mandibular  arches  arise  elongated  thickenings 


THE  STUDY  OF  PIG  EMBRYOS 


380 


that  will  become  the  body  of  the  tongue.  Between,  and  fused  to  these 
thickenings,  is  the  temporary  tiiberculum  impar.  The  opening  of  the 
thyroglossai  duct,  between  the  tuberculum  impar  and  the  second  arch,  is 
obliterated  early.  A median  ridge,  or  copula,  between  the  second  arches 
represents  the  root  of  the  tongue  and  connects  the  tuberculum  impar  with 
the  epiglottis,  which  develops  from  the  bases  of  the  third  and  fourth 
branchial  arches.  On  either  side  of  the  slit-like  glottis  are  the  arytenoid 


Dorsal  Pancreas 


Lung 


Siomach 


Ventricle 
Yolk  sac 

plum  transversum 
Yolk  stalk 
Liver 


Hepatic  diverticulum 
Duodenum 


L.  genital  fold 

L.  mesonephros 


Dorsal  aorta 


Cecum 

intestine 
llantois 

Urogenital  sinus 

U reter 
Mesonephric  duct 


Umbilical  artery  (cut  away) 

Metanephros  Rectum 

Fig.  392. — Median  sagittal  dissection  of  a 10  mm.  pig  embryo.  X 10.5. 


Atrium 


Bulhus  cordis 


Melrncephdlon 
Tela  chorioidea 


Ncurumercs  of  myelencc 

Notochord 
Tongue 


Spi)ial  cord 

Esophagus 
Trachea 


Mesencephalon 


Post,  lobe  of  hypophysis 


Optic  recess 

Telencephalon 
Ant.  lobe  of  hypophysis 


folds  of  the  larynx.  The  pharyngeal  pouches  are  now  larger  than  in  the 
6 mm.  pig  (Fig.  393).  The  first  pouch  persists  as  the  auditory  tube  and 
middle  ear  cavity,  the  ‘closing  plate’  beetwen  it  and  the  first  branchial 
groove  forming  the  tympanic  membrane . The  second  pouch  later  largely 
disappears;  about  it,  develops  the  palatine  tonsil.  The  third  pouch  is 
tubular,  directed  at  right  angles  to  the  pharynx,  and  meets  the  ectoderm 


THE  ANATOMY  OF  TEN  TO  TWELVE  MM.  PIG  E]MBRYOS 


381 


to  form  a closing  plate.  The  ventral  diverticulum  of  the  third  pouch  is  the 
anlage  of  the  thymus  gland;  its  dorsal  diverticulum  forms  a parathyroid 
gland.  The  fourth  pouch  is  smaller  and  its  dorsal  diverticulum  gives  rise 
to  a second  parathyroid  body ; the  ventral  diverticulum  is  a rudimentary 
thymus  anlage.  A tubular  outgrowth,  caudal  to  the  fourth  pouch,  is 
regarded  as  a fifth  pharyngeal  pouch  in  human  embryos;  it^forms  the 
ultimohranchial  body  on  each  side.  The  thyroid  gland,  composed  of 


Ga7ig.  nn.  7 and  8 
Olocyst 

Pharyngeal  pouch  2 
Gang,  jtigiilare  n.  10 

Aortic  arch  j 


Gang.  n.  5 


Pharyngeal  pouch  j 

Caudal  root  n. 
hypoglossus 
Gang.  Fror' 

Aortic  arch  4 
Gang.  cerv.  i 

Pharyngeal  pouch  4 

Aortic  arch  6 

R.  descending  aorta 

Esophagus 

Trachea 

Vertebral  artery 
Subclavian  artery 
R.  lung 


R.  atrium 

Stomach 


Dorsal  pancreas 

Vitelline  artery 

Ventral  pancreas 

Descending  aorta 


Post,  lobe  of  hypophysis 

Ant.  lobe  of  hypophysis 

Eye 

haryngeal  pouch  i 
Maxillary  process 
Thyroid  gland 
Pulmonary  artery 
Aorta 
Yolk  sac 
R.  ventricle 

Septum 
iransversum 

Liver 

Hepatic 
diverticidim 
Cloaca 
Allantois 
Rectum 

U refer 


Notochord 


Metanephros 
Umbilical  artery 


Mesonephric  duct 
Cephalic  limb  intest,  loop 


Fig.  393. — Reconstruction  of  a 10  mm.  pig  embryo.  X 10.  The  veins  are  not  included; 
broken  lines  indicate  the  outline  of  the  left  mesonephros  and  the  positions  of  the  limb  buds. 


branched  cellular  cords,  is  located  in  the  midplane  between  the  second 
and  third  branchial  arches  (Fig.  393). 

Trachea  and  Lungs. — Caudal  to  the  fourth  pharyngeal  pouches,  the 
esophagus  and  trachea  separate  and  form  entodermal  tubes  (Figs.  392 
and  393).  Cephalad  of  the  point  where  the  trachea  bifurcates  to  form 
rhe  primary  bronchi,  there  appears  on  its  tight  side  the  tracheal  bud  of  the 


382 


THE  STUDY  OF  PIG  EMBRYOS 


upper  lobe  of  the  right  lung  (Fig.  394).  This  bronchial  bud  is  developed 
only  on  the  right  side  and  appears  in  embryos  of  8 to  9 mm.  Two  second- 
ary bronchial  buds  arise  from  the  primary  bronchus  of  each  lung,  and 
form  the  anlages  of  the  symmetrical  lobes  of  each  lung. 

Esophagus  and  Stomach. — -The  esophagus  extends  as  a narrow  tube 
past  the  lungs,  where  it  dilates  into  the  stomach.  The  stomach  is  wide 
from  its  greater  to  its  lesser  curvature  and  shows  a cardiac  diverticulum. 
As  a whole,  it  has  rotated  so  the  original  dorsal  border,  now  the  greater 


Lateral  nasal  process 
Lacrimal  groove 

Maxillary  process 
Mandibular  process 

Cervical  sinus 
T rachea 

Tracheal  lung  hud 
Upper  limb  hud 
Septum  transversum 

Hepatic  diverticulum 
Yolk  sac 
Volk  stalk 


.1  llanlois 
R.  umbilical  artery 


Lou’er  limb  bud 

Mesonephric  duct 


Olfactory  pit 
Eye 

Median  nasal  process 

Branchial  arch  2 
Branchial  arch  3 
Branchial  arch  4 


L.  lung 

Esophagus 

Stomach 

Mesonephric  duct 
Ventral  pancreas 
Mesonephros 
Cephalic  limb  of  intestine 

limb  of  intestine 

Rectum 
elanephros 


Spinal  cord 

Fig.  394. — Ventral  dissection  of  a 9 mm.  pig  embryo.  X 9.  The  head  is  bent  dorsad. 


curvature,  lies  to  the  left,  the  ventral  border  (lesser  curvature)  to  the 
right  (Fig.  408). 

Intestine. — -The  pyloric  end  of  the  stomach  opens  into  the  duodenum, 
from  which  the  liver  and  pancreas  develop.  The  liver,  with  its  four  lobes, 
fills  in  the  space  between  the  heart,  stomach,  and  duodenum  (Figs.  390 
and  392).  Extending  from  the  right  side  of  the  duodenum  along  the  dorsal 
and  caudal  surface  of  the  liver  is  the  hepatic  diverticulum.  It  lies  to  the 
right  of  the  midplane  and  its  extremity  is  the  saccular  gall  bladder.  Several 


THE  ANATOMY  OF  TEN  TO  TWELVE  MM.  PIG  EMBRYOS 


383 


ducts  connect  the  diverticulum  with  the  liver  cords.  One  of  these  persists 
as  the  hepatic  duct  which  joins  the  cystic  duct  of  the  gall  bladder.  The 
portion  of  the  diverticulum  proximal  to  this  union  becomes  the  common 
bile  duct,  or  ductus  choledochus.  The  ventral  pancreas  arises  from  the 
common  bile  duct,  near  its  point  of  origin  (Fig.  393).  It  is  directed  dorsad 
and  caudad,  to  the  right  of  the  duodenum.  The  dorsal  pancreas  arises 
more  caudally  from  the  dorsal  wall  of  the  duodenum,  and  its  larger,  lobu- 
lated  body  grows  dorsally  and  cranially  (Figs.  393,  397  and  410).  Between 
the  pancreatic  anlages  courses  the  portal  vein.  In  the  pig,  the  duct  of  the 
dorsal  pancreas  persists  as  the  functional  duct. 

Caudal  to  the  duodenum,  the  intestinal  loop  extends  well  into  the 
umbilical  cord  (Figs.  392  and  393).  At  the  bend  of  the  intestinal  loop 
is  the  slender  yolk  stalk.  The  cephalic  limb  of  the  intestine  lies  to  the 
right,  owing  to  the  rotation  of  the  loop.  The  small  intestine  extends 
as  far  as  a slight  enlargement  of  the  caudal  limb  of  the  loop,  the 
anlage  of  the  cecum  (Fig.  392).  This  anlage  marks  the  beginning  of  the 
large  intestine  (colon  and  rectum).  The  cloaca  is  now  nearly  separated 
into  the  rectum  and  urogenital  sinus;  the  cavity  of  the  former  is  almost 
occluded  by  epithelial  cells. 

Coelom  and  Mesenteries. — The  coelom  is  still  a communicating  system 
which  includes  the  single  pericardial  cavity,  the  paired  pleural  canals,  and 
the  common  peritoneal  chamber.  Between  the  heart  and  liver  is  a promi- 
nent partition,  the  septum  transversum,  which  will  form  most  of  the  dia- 
phragm (Fig.  392).  The  attachment  of  the  liver  to  it  is  retained  as  the 
coronary  and  triangular  ligaments.  The  double  sheet  of  splanchnic  meso- 
derm that  serves  as  the  dorsal  mesentery  follows  the  intestinal  loop  into 
the  umbilical  cord  (Fig.  393).  The  primitive  ventral  mesentery  has 
mostly  disappeared  except  at  the  level  of  the  liver  where  it  will  persist  as 
the  lesser  omentum  a.ml  jalciform  ligament. 

Urogenital  System. — The  mesonephros  is  much  larger  and  more 
highly  differentiated  than  in  the  6 mm.  embryo  (Figs.  390  and  394). 
Along  the  middle  of  its  ventro-median  surface,  the  genital  j old  is  now  more 
prominent  (Fig.  392).  In  a ventral  dissection  (Fig.  394)  the  course  of  the 
mesonephric  ducts  may  be  traced.  They  open  into  the  urogenital  sinus, 
which  also  receives  the  allantoic  stalk  (Fig.  392). 

The  meianephros,  or  permanent  kidney  anlage,  lies  just  mesial  to  the 
umbilical  arteries  where  they  leave  the  aorta  (Fig.  393).  Its  epithelial 
portion,  derived  from  the  mesonephric  duct,  is  differentiated  into  a 
proximal,  slender  duct,  the  ureter,  and  into  a distal,  dilated  pelvis.  From 
this  grow  out  later  the  calyces  and  collecting  tubules  of  the  kidney.  Sur- 
rounding the  pelvis  is  a layer  of  condensed  mesenchyme,  or  nephrogenic 
tissue,  which  is  the  anlage  of  the  secretory  tubules  of  the  kidney. 


384 


THE  STUDY  OF  PIG  EMBRYOS 


Vascular  System. — The  Heart. — In  Fig.  395  the  cardiac  cham- 
bers of  the  right  side  are  opened.  The  septum  primuni,  between  the  atria, 
is  perforated  dorsad  and  cephalad  by  the  foramen  ovale.  The  inferior 
vena  cava  is  seen  opening  into  the  sinus  venosus,  which  in  turn  communicates 
with  the  right  atrium  through  a sagittal  slit  guarded  by  the  right  and  left 
valves  of  the  sinus  venosus.  The  right  valve  is  the  higher  and  its  dorsal 
half  is  cut  away.  The  valves  were  united  cephalad  as  the  septum  spurium. 
Between  the  left  valve  and  the  septum  primum,  the  sickle-like  fold  of  the 
septum  secundum  is  forming;  the  fusion  of  these  three  components  gives 
rise  later  to  the  adult  atrial  septum.  The  aortic  bulb  is  divided  distally 
into  the  aorta  and  the  pulmonary  artery,  the  latter  connecting  with  the 
sixth  (apparent  fifth)  pair  of  aortic  arches.  Proximally,  the  bulb  is 
undivided.  The  interventricular  septum  is  complete  except  for  the  inter- 
ventricular  foramen  which  leads  from  the  left  ventricle  into  the  aortic  side 
of  the  bulb.  Of  the  bulbar  swellings  which  divide  the  bulb  into  aorta 
and  pulmonary  trunk,  the  left  joins  the  interventricular  septum,  while 
the  right  extends  to  the  endocardial  cushion.  These  folds  eventually 
fuse,  and  the  partitioning  of  the  ventricular  portion  of  the  heart  is  completed. 

Sept.  II  R.  atrium 


foramen 


Fig.  395. — Heart  of  a 12  mm.  pig  embryo,  dissected  from  the  right  side. 

The  endocardium,  at  the  atrio-ventricular  foramina,  is  already  undermined 
to  form  the  anlages  of  the  tricuspid  and  bicuspid  valves.  From  the  caudal 
wall  of  the  left  atrium  there  is  given  off  a single  pulmonary  vein. 

The  Arteries. — As  seen  in  Fig.  393,  the  first  two  aortic  arches  have 
disappeared.  Cranial  to  the  third  arch,  the  ventral  aortae  become  the 
external  carotids.  The  third  aortic  arches  and  the  cephalic  portions  of  the 
descending  aortae  constitute  the  internal  carotid  arteries.  The  ventral 
aortae,  between  the  third  and  fourth  aortic  arches,  persist  as  the  common 
carotid  arteries.  The  descending  aortae  in  the  same  region  are  slender 
and  eventually  atrophy.  The  fourth  aortic  arch  is  largest,  and,  on  the 


THE  ANATOMY  OF  TEN  TO  TWELVE  MM.  PIG  EMBRYOS  385 

left  side,  will  form  the  aortic  arch  of  the  adult;  from  the  right  fourth  arch 
caudad,  the  right  descending  aorta  is  smaller  than  the  left.  Opposite 
the  eighth  segment,  the  two  aortae  unite  and  continue  caudally  as  the 
median  dorsal  aorta.  The  sixth  aortic  arches  (cf.  p.  362)  are  connected 
with  the  pulmonary  trunk,  and  from  them  arise  small  pulmonary  arteries 
to  the  lungs.  Intersegmental  arteries  extend  dorsad,  six  pairs  from  the 
descending  aortae,  others  from  the  dorsal  aorta.  From  the  seventh  pair, 
which  originate  just  where  the  descending  aortae  fuse,  the  subclavian  arteries 

Ant.  cardinal  vein 


Ant.  cardinal  vein 


Subcardinal 

anastomosis 


Post,  cardinal  vein 


Fig.  396. — Reconstruction  of  a 12  mm.  pig  embryo,  to  show  the  veins  and  heart  from  the 

left  side  (after  Lewis).  X 9. 


pass  off  to  the  upper  limb  buds  and  the  vertebral  arteries  to  the  head.  The 
latter  are  formed  by  a longitudinal  anastomosis  between  the  first  seven 
pairs  of  intersegmental  arteries  on  each  side,  after  which  the  stems  of  the 
first  six  pairs  atrophy. 

Ventro-lateral  arteries  from  the  dorsal  aorta  supply  the  mesonephros 
and  genital  ridge  (Fig.  393).  Ventral  arteries  form  the  cceliac  artery  to 


R.  atrium 


R.  umbilical 

vein 


Subcardinal  vein 


Ventricle 

Common 
hepatic  vein 


Ductus  venosiis 


Portal  vein 


Common  cardinal 
vein 


Subclavian  vein 


ost.  cardinal  vein 


25 


386 


THE  STUDY  OF  PIG  EMBRYOS 


the  stomach  region,  the  vitelline,  or  superior  mesenteric  artery,  to  the 
small  intestine,  and  the  inferior  mesenteric  artery  to  the  large  intestine. 

The  umbilical  arteries  now  arise  laterally  from  secondary  trunks 
which  persist  as  the  common  iliac  arteries. 

The  Veins. — The  veins  of  the  head  drain  into  the  anterior  cardinal  veins, 
which  become  the  internal  jugular  veins  of  the  adult  (Fig.  396).  After 
receiving  the  newer  external  jugular  veins  and  the  subclavian  veins  from  the 
ujiper  limb  buds,  the  anterior  cardinals  open  into  the  common  cardinal  veins 
(ducts  of  Cuvier)  wdiich  in  turn  empty  into  the  right  atrium. 

The  posterior  cardinal  veins  arise  in  the  caudal  region,  counse  dorsal 
to  the  mesonephroi,  and  drain  the  mesonephric  sinusoids.  The  mh- 


I 


I 


'TT !! 

r iii 
'>  III 


Notochord 
Fharyn.x 


R.  ant.  cardinal  vein 


Pericardial  cavity 


Sinus  venosiis 


Inf.  vena 

.Sin  usoidal 
connecti 

Portal  vein 
Ventral  pancreas 
Cecum 

L.  vitelline  vei 

Small  intestine 

Sup.  mesenteric 

R.  umhilical  vein 
R.  umhilical 


Upper  limb 

Common  car- 
dinal vein 


vows  us 


Allantois 


Ant.  cardinal  vein 
Esophagus 


T rachca 


Liver 

Pyloric  stomach 
Hepatic  diverticulum 
Dorsal  pancreas 
Duodenum 
L.  umbilical  vein 


Fig.  397. — Ventral  reconstruction  of  a 10  mm.  pig  embryo,  to  show  the  umbilical  and  vitel- 
line veins.  X 15.  In  the  small  orientation  figure  (cf.  Fig.  393)  the  various  planes  are  indi- 
cated by  broken  lines — * *. 


cardinal  veins  anastomose  just  caudal  to  the  origin  of  the  superior  mesen- 
teric artery,  and  the  posterior  cardinals  are  interrupted  at  this  level.  The 
proximal  portions  of  the  posterior  cardinals  open  into  the  common  cardinal 
veins,  as  in  the  6 mm.  embryo.  Of  the  two  subcardinal  veins,  the  right 
has  become  very  large  through  its  connection  with  the  right  posterior 
cardinal  vein  and  the  common  hepatic  vein,  and  now  forms  the  middle 
portion  of  the  inferior  vena  cava. 


I 

II 

in 

II 


II 

IB 


I 


i| 


THE  ANATOMY  OF  TEN  TO  TWELVE  MM.  PIG  EMBRYOS  387 

The  umbilical  veins  (Figs.  396  and  397)  anastomose  in  the  umbilical 
cord,  separate  on  entering  the  embryo,  and  course  cephalad  in  the  ventro- 
lateral body  wall  of  each  side  to  the  ventral  lobes  of  the  liver.  The  left 
vein  is  much  the  larger,  and,  after  entering  the  liver,  its  course  is  to  the 
right  and  dorsad.  After  connecting  with  the  portal  vein,  it  continues 
as  the  ductus  venosus  and  joins  the  proximal  end  of  the  inferior  vena  cava. 
The  smaller,  right  umbilical  vein  enters  the  liver  and  breaks  up  into 
sinusoids.  It  soon  atrophies,  while  the  left  vein  persists  until  after  birth. 

The  Vitelline  Veins. — Of  these,  a distal  portion  of  the  left  and  a 
proximal  portion  of  the  right  are  persistent.  The  tw'o  fused  vessels 


Met  encephalon 


Fig.  398. — Reconstruction  of  a 10  mm.  pig  embryo  (cf.  Fig.  393).  X 8.  The  numbered  lines 
indicate  the  levels  of  transverse  sections  shown  as  Figs.  399-413. 

course  from  the  yolk  sac  cephalad  of  the  intestinal  loop.  Near  a dorsal 
anastomosis  between  the  right  and  left  vitelline  veins,  just  caudal  to  the 
duct  of  the  dorsal  pancreas,  the  left  receives  the  superior  mesenteric  vein,  a 
new  vessel  arising  in  the  mesentery  of  the  intestinal  loop.  Cranial  to  this 
junction,  the  left  vitelline  (with  the  dorsal  anatomosis  and  the  proximal 
portion  of  the  right  vitelline  vein)  forms  the  portal  vein,  which  gives  off 
branches  to  the  hepatic  sinusoids  and  connects  with  the  left  umbilical  vein. 

Transverse  Sections  of  a Ten  Mm.  Pig  Embryo 
The  more  important  levels,  as  indicated  by  guide  lines  on  Fig.  398, 
are  illustrated  and  described.  These  are  useful  for  the  identification  of 


388 


THE  STUDY  OF  PIG  EMBRYOS 


organs,  but  the  student  must  interpret  his  sections  with  reference  to  the 
dissections  and  reconstructions,  and  especially  Fig.  393.  All  the  sections 
figured  are  drawn  from  the  cciihalic  surface;  accordingly,  the  right  side  of 
the  emliryo  is  at  the  reader’s  left. 

Sections  through  the  Cephalic  Flexure. — Due  to  the  flexed  head,  the  sections 
first  encountered  pass  through  the  mesencephalon  and  metencephalon.  Soon,  the  former 
becomes  continuous  with  the  thin-roofed  myelencephalon,  and  then  the  mid-brain  gives 
way  to  the  dieiicephalon  as  the  brain  becomes  cut  twice.  At  the  latter  level,  several 


Gang,  jugidare  n.  10 

Gang,  aiusticum  n.  S 

Mandibular  ramus  n.  5 
Maxillary  ramus  n.  5 

A)it.  lobe  of  hypophysis 
Lens  vesicle 
Third  vcnlriclc  of  dicncephalon 


N.  glossopharyngeus 

. geniculi.  V 7 
N.  abducens 
Basilar  artery 
inns  cavernosus 
hit.  carotid  artery 

ptic  vesicle 

interventriculare 


Lat.  ventricle  of  telencephalon'' 

Fig.  399. — Transverse  section  through  the  eyes  and  otocysts  of  a 10  mm.  pig  embryo.  X 22.5. 


N.  accessorius 


Fourth  ventricle 


Wall  of  myelencephalon 


interesting  structures  may  be  identified  in  the  mesenchyme  between  the  two  por- 
tions of  the  brain.  In  the  midplane,  but  nearer  the  myelencephalon,  is  the  single 
basilar  artery;  ventro-lateral  to  the  diencephalon  are  the  paired  internal  carotids  (cf. 
Fig.  399).  A little  cephalad  in  the  series,  the  three  vessels  unite  at  the  location  of  the 
future  arterial  circle  (of  Willis).  About  halfway  between  the  midplane  and  the  lateral 
wall  appear  branches  of  the  anterior  cardinal  veins,  and  the  oculomotor  and  trochlear 
nerves.  Of  the  two  nerves,  the  trochlear  is  smaller  and  slightly  more  laterad,  but  in 
some  series  it  is  inconspicious.  The  origin  and  relations  of  these  nerves  show  plainly 
in  Fig.  390. 


THE  ANATOMY  OF  TEN  TO  TWELVE  MM.  PIG  EMBRYOS 


389 


Section  through  the  Trigeminal  Nerve  and  Apex  of  the  Otocyst. — The  general  appear- 
ance is  like  Fig.  377  of  the  6 mm.  stage.  The  diciicephalon  is  somewhat  oblong  in  outline; 
the  niyelencepbalon  is  sectioned  lengthwise  and  its  wall  still  shows  the  neuromeric  notches. 
At  the  beginning  of  the  hind-brain  is  the  large  scmilimar  ganglion  of  the  trigeminal  nerve; 
from  its  median  side  nerve  fibers  join  the  brain  wall.  This  ganglion,  always  situated  at 
the  angle  of  the  myelencephalon,  constitutes  one  of  the  most  important  landmarks  of  the 
embryonic  head.  Midway  along  each  side  of  the  myelencephalon  will  be  seen  the  apex  of 
an  otocyst,  and  mesial  to  it  the  endolyniph  duct;  the  duct  opens  into  the  otocyst  at  a slightly 
lower  level.  Sections  cut  in  the  plane  of  this  series  do  not  include  other  structures  of 


Olfactory  pit 


Fig.  400. — Transverse  section  through  the  first  and  second  pharyngeal  pouches  of  a 10  mm.  pig 

embryo.  X 22.5. 


A.  accessorius 
Gang.  Froriep 
Myelencephalon 


Basilar  artery 


Notochord 


Gang,  petrosum  n.  g 
Pharyngeal  pouch  2 


Pharyngeal  pouch  i 


Oral  cavity 


Neural  cavity 


Roots  of  n.  hypoglossus 


Int.  jugular  vein 

Nn.  vagus  cl  accessorius 
Descending  aorta 

N.  facialis 
Branchial  arch  2 
Tongiie 

Mandible 
Oral  opening 


process 


importance  except  those  already  described  as  occurring  in  the  mesenchyme  between  the 
two  portions  of  the  brain. 

Section  through  the  Eyes  and  Otocysts  (Fig.  390). — The  brain  is  sectioned  twice, 
lengthwise  through  the  myelencephalon,  transversely  through  the  fore-hrain.  The  brain 
wall  shows  differentiation  into  three  layers:  (i)  an  inner  ependymal  layer,  densely  cellular; 
(2)  a middle  mantle  layer  of  nerve  cells  and  fibers;  (3)  an  outer  marginal  layer,  chiefly 
fibrous.  These  same  three  layers  are  developed  in  the  spinal  cord.  A thin,  vascular  layer, 
differentiated  from  the  mesenchyme,  surrounds  the  brain  wall  and  is  the  anlage  of  the  pia 


390 


THE  STUDY  OF  PIG  EMBRYOS 


mater.  The  myelencephalon  exhibits  three  neuromeres  in  this  section.  The  telencephalon 
is  repre.sented  by  the  paired  cerebral  hemispheres,  their  cavities,  the  lateral  ventricles,  con- 
necting through  the  interventricular  joramina  with  the  third  ventricle  of  the  diencephalon. 
Close  to  the  ventral  wall  of  the  diencephalon  is  a section  of  the  epithelial  lobe  of  the  hypo- 
physis (Rath he’s  pouch),  near  which  are  the  internal  carotid  and  basilar  arteries.  Lateral 
to  the  diencephalon  are  the  optic  cup  and  lens  vesicle  of  the  eye,  which  are  sectioned  caudal 
to  the  optic  stalk.  The  outer  layer  of  the  optic  cup  forms  the  thin  pigment  layer;  the  inner, 
thicker  layer  is  the  nervous  layer  of  the  retina.  The  lens  is  now  a closed  vesicle,  distinct 
from  the  overlying  corneal  ectoderm. 

The  large  vascular  spaces  are  the  cavernous  sinuses,  which  drain  into  the  internal 
jugular  veins  (anterior  cardinals).  Transverse  sections  of  the  maxillary  and  mandibular 
branches  of  the  n.  trigeminus  may  be  seen;  the  n.  abducens  is  sectioned  longitudinally. 


Mandible 


Spinal  ganglion 


Spinal  cord 


Int.  jugular  vein 
N.  hypo  gloss  us 
Gang,  nodosum  n lo 

Pharyngeal  pouch  j 

ortic  arch  j 
Thyroid  ant  age 


Median  nasal  process 

epithelium 
nasal  process 


Notochord 


Ext,  branch  n.  accessotius 


Epiglottis 
Branchial  arch 


Branchial  arch  2 


Olfactory  pit 


Fig.  401. — Transverse  section  through  the  third  pharyngeal  pouches  and  olfactory  pits  of  a 10 

mm.  pig  embryo.  X 22.5. 


In  front  of  the  otocyst  occur  the  geniculate  and  acoustic  ganglia  of  the  nn.  facialis  and 
acustieus.  The  wall  of  the  otocyst  forms  a sharply  defined  epithelial  layer  which  makes  a 
convenient  landmark  in  identifying  ganglia  and  nerves.  Caudal  to  the  otocyst,  the  n. 
glossopharyngeus  and  The  jugular  ganglion  of  the  vagus  are  cut  transversely  while  the  trunk 
of  the  n.  accessorius  is  sectioned  lengthwise. 

Section  through  the  First  and  Second  Pharyngeal  Pouches  (Fig.  400). — The  end  of 
the  head,  with  parts  of  the  telencephalon  and  olfactory  pits,  is  now  distinct  from  the  rest  of 


THE  ANATOMY  OP  TEN  TO  TWELVE  MM.  PIG  EMBRYOS 


391 


the  section.  Higher  in  the  series,  Rathke's  pouch  opens  into  the  stomodeum  between  the 
jaws;  this  level  is  at  the  actual  oral  opening.  The  pharynx  shows  portions  of  the  first  and 
second  pharyngeal  pouches.  Opposite*the  first  pouch,  externally,  is  the  first  branchial  groove. 
A shaving  from  the  tubcrcidiim  impar  of  the  tongue  lies  the  midplane  in  the  pharyngeal 
cavity.  The  neural  tube  is  sectioned  dorsally  at  the  level  of  Froriep  's  ganglion.  Between 
the  neural  tube  and  the  pharynx  may  be  seen  on  each  side  the  several  root  fascicles  of  the 
n.  hypoglossus,  the  fibers  of  the  nn.  vagus  and  accessorius,  and  the  petrosal  ganglion  of  the 
n.  glossopharyngeus.  Mesial  to  the  ganglia  are  the  descending  aortce,  andlateral  to  the  vagus 
is  the  internal  jugular  vein. 

Section  through  the  Third  Pharyngeal  Pouches  (Fig.  401). — The  tip  of  the  head  is 
now  small  and  includes  on  either  side  the  deep  olfactory  pits  lined  with  thickened  olfactorv 


Spinal  ganglion 


Spinal  cord 


R.  descending  aorta 

N.  vagus 
Tiacheal  groove 


R.  atrium 


Aorta 


Pulmonary  artery 
R.  ventricle 


L.  descending  aorta 
. jugular  vein 


Pharyngeal  pouch  4 


L.  atrium 


cricardial  cavity 


L.  ventricle 


Fig.  402. — Transverse  section  through  the  fourth  pharyngeal  pouches  of  a 10  mm.  pig  embryo. 

X 22.5. 


epithelium.  Each  pit  is  bordered  by  a lateral  and  median  nasal  process.  The  first,  second, 
and  third  branchial  arches  show;  the  first  pair  are  fused  as  the  mandible,  the  third  pair  are 
slightly  sunken  in  the  cervical  sinus.  The  dorsal  diverticula  of  the  third  pharyngeal  pouches 
extend  toward  the  ectoderm  of  the  third  branchial  groove.  The  ventral  diverticula,  or 
thymic  anlages,  may  be  traced  caudad  in  the  series.  The  floor  of  the  pharynx  is  sectioned 
through  the  epiglottis.  Ventral  to  the  pharyn.x  are  portions  of  the  third  aortic  arches  and 
the  solid  cords  of  the  thyroid  gland.  Dorsally,  the  section  passes  through  the  spinal  cord 
and  first  pair  of  cervical  ganglia.  Betw'een  the  cord  and  pharynx,  named  in  order,  are  the 
internal  jugular  veins,  the  hypoglossal  nerve,  and  the  nodose  ganglion  of  the  vagus.  Lateral 
to  the  ganglion  is  the  external  branch  of  the  n.  accessorius,  and  mesial  to  the  ganglia  are 
the  small  descending  aortce. 


392 


THE  STUDY  OF  PIG  EMBRYOS 


Section  through  the  Fourth  Pharyngeal  Pouches  (Fig.  402). — This  region  is  marked 
by  the  disappearance  of  the  head,  and  the  appearance  of  the  heart  in  the  pericardial  cavity. 
The  tips  of  the  atria  are  sectioned  as  they  project  on  either  side  of  the  biilbus  cordis.  The 
bulbus  is  divided  into  the  aorta  and  pulmonary  artery,  the  latter  connected  with  the  right 
ventricle,  which  has  spongy  muscular  walls.  The  crescentic  pharynx  is  continued  laterally 
as  the  small  fourth  pharyngeal  pouches,  and  into  its  midventral  wall  opens  the  vertical  slit  of 
the  trachea.  A section  of  the  vagus  complex  is  located  between  the  descending  aorta  and 
the  internal  jugular  vein.  At  this  level,  the  jugular  vein  receives  the  linguo-facial  vein. 
The  left  descending  aorta  is  larger  than  the  right  in  anticipation  of  its  conversion  into  the 
permanent  arch  of  the  aorta.  The  ventral  aorta  may  be  traced  cranially  in  the  series  to 


Fig.  403. — Transverse  section  through  the  sixth  pair  of  aortic  arches  and  bulbus  cordis  of  a 

10  mm.  pig  embryo.  X 22.5. 

the  fourth  aortic  arches.  The  pulmonary  artery,  if  followed  caudad,  connects  with  the 
sixth  aortic  arches  as  in  Fig.  403. 

Section  through  the  Sixth  Aortic  Arches  (Fig.  403). — The  sixth  aortic  arch  (see  p.  362) 
is  complete  on  the  left  side  of  the  embryo.  From  these  pulmonary  arches  small  pulmonary 
arteries  may  be  traced  caudad  in  the  series  to  the  lung  anlages.  The  esophagus,  now  sep- 
arate from  the  trachea,  forms  a curved  horizontal  slit.  All  four  chambers  of  the  heart  are 
represented,  but  the  aorta  and  pulmonary  artery  are  divided  incompletely  by  the  right  and 
left  bulbar  swellings,  or  folds.  The  vagus  nerves  are  prominent.  Ventro-lateral  to  the 
spinal  cord  are  diffuse  myotome  masses. 

Section  through  the  Sinus  Venosus  and  the  Heart  (Fig.  404). — The  section  is  marked 
by  the  symmetrically  placed  atria  and  ventricles  of  the  heart  and  by  the  presence  of  the 
upper  limb  buds.  Dorsal  to  the  atria  are  the  common  cardinal  veins,  the  right  vein  forming 
part  of  the  sinus  venosus,  the  left  connecting  at  a lower  level.  The  sinus  venosus  drains 


THE  ANATOMY  OF  TEN  TO  TWELVE  MM.  PIG  EMBRYOS 


393 


into  the  right  atrium  through  a sht-like  opening  in  the  dorsal  and  caudal  atrial  wall.  The 
opening  is  guarded  by  the  right  and  left  valves  of  the  sinus  venosus,  which  project  into  the 
atrium.  The  septum  primiim  completely  divides  the  right  and  left  atria  at  this  level,  which 
is  caudal  to  the  foramen  ovale  and  the  atrio-ventricular  openings.  The  septum  joins  the 
fused  endocardial  cushions.  Note  that  the  esophagus  and  trachea  are  now  tubular  and 
that  the  left  descending  aorta  is  much  larger  than  the  right.  Around  the  epithelium  of 
both  trachea  and  esophagus  are  condensations  of  mesenchyme,  from  which  their  outer 
layers  are  differentiated;  laterad  in  this  mass  lie  the  vagi. 

Section  through  the  Foramen  Ovale  of  the  Heart  (Fig.  405).- — The  level  of  this  sec- 
tion is  cranial  to  that  of  the  previous  figure  and  shows  the  septum  primum  interrupted  dor- 
sally  to  form  the  foramen  ovale.  Each  atrium  communicates  with  the  ventricle  of  the  same 


Spinal  ganglion 


Spinal  cord 


R.  descending  aorta 
Sinus  venosus 

R.  valve  of  sinus  venosus 
Pericardial 

R. 

Interventricular 


U pper  limb  bua 
Esophagus 

L.  common  cardinal  vein 
Trachea 


L.  atrium 


Endocardial  cushion 


Body  wall 


Fig.  404. — Transverse  section  through  the  sinus  venosus  and  heart  of  a 10  mm.  pig  embryo. 

X 22.5. 


side  through  the  atrio-ventricular  foramen.  Between  these  openings  is  the  endocardial  cush- 
ion, which  in  part  forms  the  anlages  of  the  tricuspid  and  bicuspid  valves.  The  atria  are 
marked  off  externally  from  the  ventricles  by  the  coronary  sulcus.  Between  the  two  ven- 
tricles is  the  interventricular  septum.  The  ventricular  walls  are  thick  and  spongy,  forming 
a network  of  muscular  cords,  or  trabeculce,  surrounded  by  blood  spaces,  or  sinusoids.  The 
trabeculce  are  composed  of  muscle  cells,  which  later  become  striated  and  constitute  the 
myocardium.  They  are  surrounded  by  an  endothelial  layer,  the  endocardium.  The  mam- 
malian heart  receives  all  its  nourishment  from  the  blood  circulating  in  the  sinusoids,  until 
later,  when  the  coronary  vessels  of  the  heart  wall  are  developed.  The  heart  is  surrounded 
by  a layer  of  mesothelium,  the  epicardium,  which  is  continuous  with  the  pericardial  meso- 
thelium  lining  the  body  wall. 


304 


THE  STUDY  OF  PIG  EMBRYOS 


Section  through  the  Liver  and  Upper  Limb  Buds  (Fig.  406). — The  section  is  marked 
by  the  presence  of  the  21  p per  limb  buds,  the  liver,  and  the  bifurcation  of  the  trachea  to 


Forame2t  ovale 
K.  atrium — -M 


R.  atrio-venlricular  foratnen 
R.  ventricle 
Interventricular  septum 


L.  atrium 
Septum  I 

Endocardial  cushion 
L.  atrio-ventriciilar  fora>2ien 

L.  ventricle 


Fig.  405. — Transverse  section  through  the  foramen  ovale  and  heart  of  a 10  mm.  pig  embryo. 

X 22.5. 


Fig.  406. — Transverse  section  through  the  liver  and  upper  limb  buds  of  a 10  mm.  pig  embryo, 
at  the  level  of  the  tracheal  bifurcation.  X 22.5. 

form  the  primary  bro)ichi  of  the  lungs.  The  limb  buds  are  composed  of  dense,  undiffer- 
entiated mesenchyme,  surrounded  by  ectoderm  which  is  thickened  at  their  tips.  The 


THE  ANATOMY  OF  TEN  TO  TWELVE  MM.  PIG  EMBRYOS 


395 


seventh  pair  of  cervical  ganglia  and  nerves  are  cut  lengthwise,  showing  the  spindle-shaped 
ganglia  with  the  dorsal  root  fibers  taking  origin  from  their  cells.  The  ventral  root  fibers 
arise  from  the  ventral  cells  of  the  mantle  layer  and  join  the  dorsal  root  to  form  the  nerve 
trunk.  On  the  right  side,  a short  dorsal  ramus  supplies  the  anlage  of  the  dorsal  muscle 
mass.  The  much  larger  ventral  ramus  unites  with  those  of  other  nerves  at  this  level  to  form 
the  brachial  plexus. 

The  descending  aorta  have  now  fused,  and  the  seventh  pair  of  dorsal  intersegmental 
arteries  arise  from  the  dorsal  aorta.  From  these  intersegmental  arteries  the  subclavian 
arteries  are  given  off  two  sections  caudad  in  the  series.  Lateral  to  the  aorta  are  the  pos- 
terior cardinal  veins.  The  esophagus,  ventral  to  the  aorta,  shows  a very  small  lumen,  while 
that  of  the  trachea  is  continued  into  the  bronchi  on  either  side.  Adjacent  to  the  esophagus 
are  the  cut  vagus  nerves.  The  lung  anlages  project  laterally  into  the  crescentic  pleural 
cavities,  of  which  the  left  is  sepaxated  from  the  peritoneal  cavity  by  the  septum  transversum. 
The  liver,  with  its  fine  network  of  trabecula  and  sinusoids,  is  large  and  nearly  fills  the 
peritoneal  cavity.  The  liver  cords  are  composed  of  liver  cells  surrounded  by  the  endothelium 


Spinal  cord 


Spinal  ganglion 


Dorsal  aorta 
Mesonephros 


R.  lung  hii 

Stomach 
Omental  bursa 
Inferior  vena  cava 


Notochord 

Sympathetic  ganglion 
Post,  cardinal  vein 

Mesonephric  tubule 

Peritoneal  cavity 
■Dorsal  lobe  of  liver 


Sinusoids  of  liver 


HivV. 


Ductus  venosus 

Fig.  407. — Dorsal  half  of  a transverse  section  through  the  lung  buds  of  a 10  mm.  pig  embryo. 

X 22.5. 


of  the  sinusoids.  Red  blood  cells  are  developing  in  the  liver  at  this  stage.  The  large  vein, 
from  the  liver  to  the  heart,  penetrating  the  septum  transversum,  is  the  proximal  portion  of 
the  inferior  vena  cava,  originally  the  right  vitelline  vein.  Ventral  to  the  bronchi  may  be 
seen  sections  of  the  pulmonary  veins. 

Section  through  the  Lung  Buds  (Fig.  407). — The  lungs  are  sectioned  through  their 
caudal  ends,  and  the  esophagus  is  just  beginning  to  dilate  into  the  stomach.  On  either  side 
of  the  circular  dorsal  aorta  are  the  mesonephroi,  while  dorso-laterally  are  sympathetic 
ganglia.  The  pleural  cavities  now  communicate  freely  on  both  sides  with  the  peritoneal 
cavity.  A section  of  the  omental  bursa  appears  as  a crescent -shaped  slit  at  the  right  of  the 
stomach.  In  the  right  dorsal  lobe  of  the  liver  is  located  the  inferior  vena  cava.  Near  the 
median  plane,  ventral  to  the  omental  bursa,  is  the  large  ductus  venosus. 

Section  through  the  Stomach  and  Mesonephros  (Fig.  408). — Prominent  in  the  body 
cavity  are  the  mesonephroi  and  liver  lobes.  The  mesonephroi  show  sections  of  coiled 
tubules  lined  wdth  cuboidal  epithelium.  Glomeruli  of  the  renal  corpuscles,  median  in 


3Q6 


THE  STUDY  OF  PIG  EMBRYOS 


position,  have  developed  as  knots  of  small  arteries  which  grow  into  the  ends  of  the  tubules. 
The  thickened  epithelium  along  the  median  and  ventral  surface  of  the  mesonephros  is  the 
anlage  of  the  genital  gland.  The  body  wall  is  thin  and  lined  with  mesothelium  continuous 
with  that  which  covers  the  mesenteries  and  organs.  The  mesothelial  layer  becomes  the 
epithelium  of  the  adult  peritoneum,  mesenteries,  and  serous  layer  of  the  viscera.  The 
stomach  lies  on  its  left  side  and  is  attached  dorsally  by  the  greater  omentum,  ventrally  to  the 
liver  by  the  lesser  omentum.  The  right  dorsal  lobe  of  the  liver  is  attached  dorsally  to  the 
right  of  the  greater  omentum.  In  the  liver,  ventral  to  this  fusion,  courses  the  inferior  vena 
cava,  and  the  connection  forms  the  caval  mesentery.  Between  the  attachments  of  the 
stomach  and  liver,  and  to  the  right  of  the  stomach,  is  the  omental  bursa.  In  the  liver,  to 


Spinal  cord 

Notochord 

Dorsal  aorta 

Caval  mesentery 
Inferior  vena  cava 

Lesser 


Falciform  ligament 


Spinal  ganglion 

Base  of  upper  limb 


Glomerulus  of 
mesonephros 

Greater  ome?itum 
Stomach 

Dorsal  lobe  of  liver 
Ductus  venosus 


Ventral  lobe  of  liver 


Fig.  408. — Transverse  section  through  the  stomach  and  mesonephros  of  a 10  mm.  pig  embryo. 

X 22.5. 


the  left  of  the  midplane,  is  the  ductus  vawsus,  sectioned  just  at  the  point  where  it  receives 
the  left  umbilical  vein  and  a branch  from  the  portal  vein.  The  ventral  attachment  of  the 
liver  later  becomes  the  falciform  ligament. 

Section  through  the  Hepatic  Diverticulum  (Fig.  409). — The  section  passes  through  the 
pyloric  end  of  the  stomach  and  duodenum,  near  the  attachment  of  the  hepatic  diverticu- 
lum. The  greater  omentum  of  the  stomach  is  larger  than  in  the  previous  section,  and  to  its 
right,  in  the  caval  mesentery,  lies  the  inferior  vena  cava.  Ventral  to  the  inferior  vena  cava 
is  a section  of  the  portal  vein.  The  ventral  and  dorsal  lobes  of  the  liver  are  now  separate, 
and,  in  the  right  ventral  lobe,  is  embedded  the  saccular  end  of  the  hepatic  diverticulum 
which  forms  the  gall  bladder.  To  the  right  of  the  stomach,  the  common  duct  segment  of  the 


THE  ANATOMY  OF  TEN  TO  TWELVE  MM.  PIG  EMBRYOS 


397 


diverticulum  is  sectioned  just  as  it  enters  the  duodenum.  Ventrally,  the  left  tiinbilical 
vein  is  entering  the  left  ventral  lobe  of  the  liver.  It  is  much  larger  than  the  right  vein, 
which  still  courses  in  the  body  wall.  On  the  left  side  of  the  embryo  the  spinal  nerve  shows, 
in  addition  to  its  dorsal  and  ventral  rami,  a sympathetic  ramus,  the  fibers  of  which  passtoa 
cluster  of  ganglion  cells  located  dorso-lateral  to  the  aorta.  These  cells  form  one  of  a pair 
of  sympathetic  ganglia.  On  each  dorso-lateral  surface  of  the  trunk  is  a thickened  ecto- 
dermal ridge,  representing  the  milk  line  from  which  mammary  glands  differentiate. 

Section  through  the  Pancreatic  and  Genital  Glands  (Fig.  410). — The  omental  bursa, 
just  above  the  level  of  this  section,  has  opened  into  the  peritoneal  cavity  through  the  epiploic 
foramen  (of  Winslow).  The  mesonephric  ducts  are  now  prominent  ventrally  in  the  meso- 
nephroi, and  along  the  mesial  surfaces  are  the  thickened  anlages  of  the  genital  glands.  The 


Spinal  cord 


R.  umbilical  vein 


L.  umbilical  vein 


Fig.  409. — Transverse  section  through  the  hepatic  diverticulum  of  a 10  mm.  pig  embryo. 


Notochord 
Milk  line 
Mesonephros 


Caval  mesentery 
Inferior  vena 
Lesser  peritoneal  sac 
Portal 

Hepatic 
{Duct  and  gall 
bladder) 


Dorsal  mesogastrium 

Mesonephric  duct 
Stomach 

Ventral  lobe  of  liver 


Sympathetic  ramus 


Dorsal  aorta 


X 22.5. 


duct  of  the  dorsal  pancreas  is  sectioned  tangentially  at  the  point  where  it  takes  origin  from 
the  d uoden  u m.  F rom  the  duct , the  lobulated  gland  may  be  traced  dorsad  in  the  mesentery. 
To  the  right  of  the  dorsal  pancreatic  duct  is  a section  of  the  ventral  pancreas,  which  may  be 
traced  cephalad  in  the  series  to  its  origin  from  the  hepatic  diverticulum.  Dorsal  to  the 
ventral  pancreas  is  a section  of  the  portal  vein.  The  inferior  vena  cava  appears  as  a vertical 
slit  in  the  caval  mesentery. 

Sections  through  the  Lower  Body. — Due  to  the  curvature  of  the  lower  half  of  the  body, 
sections  transverse  to  the  main  axis  include  two  portions  of  the  embryo,  facing  each  other 
and  connected  by  the  lateral  body  wall.  The  general  appearance  is  much  like  Fig.  385 


39^ 


THE  STUDY  OF  PIG  EMBRYOS 


of  the  6 mm.  pig.  In  these  sections,  the  intestinal  loop  may  be  found  in  the  free 
mesentery.  Besides  the  gut  and  allantois,  the  vitelline  artery  (superior  mesenteric),  umbilical 
arteries,  superior  mesenteric  vein,  and  umbilical  veins  may  be  traced  to  the  umbilical  cord. 
Vcntro-lateral  arteries  are  given  off  from  the  aorta  to  the  mesonephros.  Tracing  the 
inferior  vena  cava  caudad  from  the  level  illustrated  in  Fig.  410,  it  becomes  continuous 


Dorsal  aorta 


Inferior  vena  cava 


Post,  cardinal  vein 


Mesonephros 

L.  vitelline  vein 
Dorsal  pancreas 


R.  vitelline  or  portal  vein 
Mesonephric  duel 


Ventral  pancreas' 

Fig.  410. — Transverse  section  through  the  pancreatic  and  genital  glands  of  a 10  mm.  pig 

embryo.  X 22.5. 


with  the  right  subcardinal,  now  a component  of  the  vena  cava  (cf.  Fig.  385).  The  left 
subcardinal  is  smaller  and  the  two  connect  by  anastomoses. 

Section  through  the  Cloaca. — At  a level  intermediate  between  Figs.  410  and  41 1 (cf. 
Fig.  3q8.),  the  cloaca  and  cloacal  membrane  will  be  encountered  in  the  tail  portions  of 
sections.  These  relations  should  be  compared  with  Fig.  387  of  the  6 mm.  pig  series. 


Fig.  41 1. — Transverse  section  through  the  urogenital  sinus  and  rectum  of  a 10  mm.  pig  embryo. 

X 22.5. 


Section  through  the  Urogenital  Sinus  and  Rectum  (Fig.  41 1). — The  figure  shows  only 
the  caudal  end  of  a section,  in  the  dorsal  portion  of  which  the  mesonephroi  were  sectioned 
at  the  level  of  the  subcardinal  anastomosis.  A portion  of  the  mesentery  is  shown  with  a 
section  of  the  colon.  In  the  body  wall  are  veins  that  drain  into  the  umbilical  veins,  and  on 
each  side  are  the  umbilical  arteries,  just  entering  the  body  from  the  umbilical  cord.  Be- 


THE  ANATOMY  OF  TEN  TO  TWELVE  MM.  PIG  EMBRYOS 


399 


tween  them,  in  sections  cranial  to  this,  the  allantoic  stalk  is  located.  Here  it  has  opened 
into  the  crescentic  urogenital  sinus.  Dorsal  to  the  urogenital  sinus  (dorsal  now  being  at 
the  bottom  of  the  figure,  owing  to  the  curvature  of  the  caudal  region),  is  a section  of  the 
rectum,  separated  from  the  sinus  by  a curved  prolongation  of  the  coelom.  From  the  ends 
of  the  urogenital  sinus,  as  we  trace  cephalad  in  the  embryo  {downward  in  the  series),  are 
given  off  the  mesonephric  ducts. 


Fig.  412. — Transverse  section  through  the  lower  limb  buds  and  ureters  of  a 10  mm.  pig  embryo. 

X 22.5. 

Section  through  the  Lower  Limb  Buds  and  Origin  of  the  Ureter  (Fig.  412). — The  sec- 
tion cuts  through  the  middle  of  both  lower  limb  buds.  Mesial  to  their  bases  are  the  umbilical 
arteries,  which  lie  lateral  to  the  mesonephric  ducts.  From  the  dorsal  wall  of  the  left  mesone- 
phric duct  is  given  off  the  ureter,  or  duct  of  the  metanephros.  Tracing  the  sections  down 
in  the  series,  both  ureters  appear  as  minute  tubes  in  transverse  section.  They  soon  dilate 
to  form  the  pelvis  of  the  kidney,  at  the  level  of  Fig.  413.  Note  the  undifferentiated  mesen- 
chyme of  the  lower  limb  buds  and  their  thickened  ectodermal  tips. 


Fig.  413. — Transverse  section  through  the  metanephric  anlages  of  a 10  mm.  pig  embryo. 

X 22.5. 

Section  through  the  Metanephroi  and  Umbilical  Arteries  (Fig.  413). — The  section 
passes  caudal  to  the  mesonephric  ducts,  which  curve  along  the  ventral  surfaces  of  the 
mesonephroi  (Fig.  393).  The  umbilical  arteries  course  lateral  to  the  metanephroi.  The 
latter  consist  merely  of  the  thickened  epithelium  of  the  renal  pelvis  surrounded  by  a layer 
of  condensed  mesenchyme,  the  nephrogenic  tissue,  which  will  differentiate  into  secretory 
tubules. 


400 


THE  STUDY  OF  PIG  EMBRYOS 


Section  through  the  Vertebral  Anlages. — Near  the  caudal  end  of  the  series,  the  sections 
pass  tangentially  through  the  curved  back  of  the  embryo.  The  appearance  is  much  like 
Fig.  386.  Slightly  cephalad  of  this  particular  level,  the  aorta  is  sectioned  lengthwise,  and 
a little  higher  still  the  longitudinal  notochord  appears  surrounded  by  segmental  masses 
of  condensed  mesenchyme.  These  are  the  anlages  of  vertebvcB  differentiating  from  the 
union  of  paired  sclerotomes. 


(Cj  THE  ANATOMY  OF  AN  EIGHTEEN  MM.  PIG  EMBRYO 
The  anlages  of  the  important  organs  are  formed  in  12  mm.  embryos. 
Older  stages  are  chiefly  instructive,  therefore,  to  demonstrate  the  growth 


Metanephros 


Nerve  to  lower  limb 

Sciatic  nerve 

Fig.  414. — Lateral  dissection  of  an  18  mm.  pig  embryo.  X 8. 


Dia  phragm 
Dorsal  lobe  of  liver 


M esonephros 


R.  atrium 
R.  ventricle 

Ventral  lobe  of  liver 


U mbilical  cord 


Mesencephalon 
Cerebellum 

Gang,  genicitli  n 7 
Gang,  acusticum  n.  8 
Gang,  superius  n.  g 
Gang,  accessorius 


Gang,  jugularc  n.  10 

Gang,  petrosum  n. 
N.  hypoglossus 

N.  accessorius 
Gang.  cerv. 


N.  opticus 
Cerebrum 

Maxillary  ramus  n.  5 
Chorda  tympani  n.  7 
N.  facialis 


Brachial 


ang.  nodosum  n.  10 


N . oculomotorius 
N.  trochlearis 

Gang,  semilunare  n.  p 


Ophthalmic  ramus  n.  5 


and  differentiation  of  parts  already  present,  rather  than  the  introduction 
of  new  ones.  Dissections  show  perfectly  the  form  and  relations  of  organs. 


THE  ANATOMY  OF  TEN  TO  TWELVE  MM.  PIG  EMBRYOS 


401 


their  relative  growth,  and  changes  of  position.  Since  the  illustrations 
indicate  better  than  descriptions  the  several  structures  and  their  states  of 
development,  only  certain  features  will  be  mentioned. 

External  Form  (cf.  Fig.  414). — The  neck  and  back  are  much  straighter 
than  before,  but  the  ventral  body  is  exceedingly  convex.  The  head  is 


Isthmus  Mesencephalon 
Metencephalon 


Chorioid  plexus  of  fourth  ventricle 
Hypophysis 


Third  ventricle 


Diencephalon 


Tela  chorioidea  of  fourth  ventricle 
Myelencephalon 

Epiglottis 

Notochord 

Trachea 
Pulmonary  artery 
Wall  of  atrium 
Fora?nen  ovale 
Esophagus 
Lung 
Dorsal  aorta 

Stomach 

Intersegmental 
arteries 

Pancreas 

Common  bile  dud 

Duodenum 

/ 

Genital  fold 

Metanephr'os 

Mesonephric  duct  Ureter 

Fig.  415. — Median  sagittal  dissection  of  an  18  mm.  pig  embryo.  X 8. 


Corpus  striatum 


Cerebrum 


Semilunar  valves 
R.  ventricle 
Yolk  sac 
Diaphragm 
Yolk  stalk 


Urogenital  sinus 
Rectum 


relatively  larger,  the  umbilical  cord  smaller.  The  sense  organs  are  promi- 
nent, and  the  face,  with  snout  and  jaws,  plain.  The  branchial  grooves 
and  cervical  sinus  have  disappeared  from  the  neck.  The  limbs  show  indi- 

26 


402 


THE  STUDY  OF  PIG  EMBRYOS 


cations  of  proximal  and  distal  divisions,  and  the  hand  and  foot  are  paddle- 
like. Several  mammary  gland  anlages  occur  along  the  milk  lines,  now 
located  more  ventrad.  The  genital  tubercle  has  become  a distinct  phallus. 

Lateral  Dissection  (Fig.  414). — The  cerebral  hemispheres  are  larger 
and  the  cerebellum  is  appearing.  Beneath  the  cerebellum  is  the  pontine 
flexure  of  the  lirain,  ])ointing  ventrad.  Nerves  and  ganglia  show  clearly;  the 
brachial  and  lumbo-sacral  plexuses,  opposite  the  limbs,  are  noteworthy. 
The  liver  and  lungs  are  relatively  larger,  the  heart  and  mesonephros 
smaller. 

Median  Sagittal  Dissection  (Fig.  415). — The  corpus  striatum  has 
developed  in  the  floor  of  the  cerebral  hemisphere,  a chorioid  plexus  invades 
the  fourth  ventricle,  and  the  neural  (posterior)  lobe  of  the  hypophysis  is 


Fig.  416. — Ventral  dissection  of  a 15  mm.  jiig  embryo.  X 6.  The  heart  and  liver  have  been 
removed  and  the  lungs  are  viewed  through  the  transparent  pericardium. 

growing  into  association  with  the  detached  Rathke’s  pouch.  Sclerotomic 
anlages  of  vertebrae  condense  about  the  notochord.  The  viscera  show  only 
quantitative  changes  from  the  12  mm.  stage,  but  the  urogenital  sinus  and 
rectum  are  now  separate.  The  relation  of  the  intestine  and  its  temporary 
herniation  into  the  umbilical  cord  are  apparent. 

Ventral  Dissection  (Figs.  416,  147  and  148). — In  the  first  two  illus- 
trations the  lungs  appear  to  lie  in  the  pericardial  cavity  but  in  reality  they 
are  viewed  through  its  transparent  dorsal  wall.  The  three  figures  show 
successive  stages  in  the  growth  of  the  Mullerian  ducts  toward  the  urogeni- 
tal sinus,  and  also  in  the  lobation  of  the  lungs. 

(D)  THE  ANATOMY  OF  A THIRTY-FIVE  MM.  PIG  EMBRYO 
External  Form  (cf.  Fig.  417).— The  embryo  is  straighter  and  its  ven- 
tral surface  less  protuberant.  The  head,  with  its  prominent  snout,  is 


THE  ANATOMY  OF  TEN  TO  TWELVE  MM.  PIG  EMBRYOS 


403 


shaping  like  that  of  a lower  mammal,  and  the  neck  becomes  distinct. 
Digits  have  appeared  on  the  elongate  extremities. 

Lateral  Dissection  (Fig.  417). — The  spinal  cord  and  brain  are  relatively 
smaller,  but  the  latter  is  becoming  highly  specialized  and  folded.  The 


Dorsal  lobe  of  liver 


Lower  limb 

Metanephros 

Lumbar  gang.  ^ 

^ Sciatic  nerve 

Nerve  to  lower  limb 

Fig.  417. — Lateral  dissection  of  a 35  mm.  pig  embryo.  X 4. 


Lung 


Diaphragm 


R.  atrium 
R.  ventricle 

Ventral  lobe  of  liver 
Umbilical  cord  i 


N . hypoglossus 
Gang.  cerv.  p-8 
Gang.  thor.  l 


Semilunar  ganglion  n.  5 
Gang,  geniculi  n.  7 

Cerebellum 


ramus  n.  5 
Cerebrum 


Gang  acusticum  n.  8 
Gang  superius  n. 

Gang,  jugulare  n.  10 
Gang.  Froriep 
Auricular  r.  n.  10 
Gang.  n.  cerv.  1 
Gang  petrosum  n. 

N.  accessorius 


Hypophysis 

N.  opticus 

Lobiis  olfactorius 


Ma.xillary  ramus  n.  5 

IMand.  ramus  n.  5 
Chorda  tympani  n.  7 

N . facialis 

Gang,  nodosum  n.  10 


cerebral  hemispheres  are  large  and  olfactory  lobes  extend  forward  from 
the  rhinencephalon.  The  body  of  the  embryo  elongates  faster  than  the 
spinal  cord,  so  that  the  spinal  nerves,  at  first  directed  at  right  angles, 
course  obliquely  in  the  lumbo-sacral  region.  Note  especially  how  the 


404 


THE  STUDY  OF  PIG  EMBRYOS 


viscera  have  receded  caudad  (cf.  Figs.  367  and  390)  and  how  the  liver  domi- 
nates the  abdomen.  The  kidney  is  exceptional  in  that  it  shifts  cephalad. 

Median  Sagittal  Section  (Fig.  418). — New  features  of  the  brain  are 
the  olfactory  lobes,  the  chorioid  plexus  of  the  third  and  lateral  ventricles, 
the  thalami,  the  epiphysis,  and  consolidated  hypophysis.  The  primitive 
mouth  cavity  is  now  divided  by  the  palatine  folds  into  upper  nasal  passages 


Mcsnicc 

Peduncidus  cerebri 


CcrehcUitm 

Chorioidal  plexus,  ventricle  4 

Tela  of  ve>ilriele  4 
M yelencephalon 


Epiglottis 

Esophagus 
Spinal  cord 
Trachea 

Aorta 
R.  atrium 
R.  bronrhus 
Dorsal  aorta 
Inf.  vc)ia  cava 
Stomach 
Pancreas 
Suprarenal  gland 
Genital  gland 
Duodenum 


Epiphysis  Thalamus 

I'cla  chorioidea 


Lat.  chorioid  plexus 

or  pus  striatum 

Hypophysis 
Lohus  olfactorius 
Turbinate  anlage 

Palate 
Tongue 

Pulmonary  artery 
Ventricle 


Gall  bladder 
Small  intestine 


Urethra 


Metanephros 

Colon 

L.  mesonephric  duct 

Urogenital  sinus  with  mesonephric  duct  Rectum 

Fig.  418. — Median  sagittal  dissection  of  a 35  mm.  embryo.  X 4- 


and  lower  oral  cavity.  Of  the  viscera,  the  distinct  genital  and  suprarenal 
glands  and  the  enlarged  metanephros  command  attention,  as  does  the  coil- 
ing of  the  intestine.  The  ureter  has  acquired  a separate  opening  at  the 
base  of  the  bladder,  and  the  urethra  may  be  traced  into  the  phallus. 


METHODS  FOR  THE  DISSECTION  OF  PIG  EMBRYOS 


405 


(E)  METHODS  FOR  THE  DISSECTION  OF  PIG  EMBRYOS 

A prominent  feature  of  the  laboratory  manual  prepared  by  Professor 
C.  W.  Prentiss  was  the  emphasizing  of  dissections  as  an  aid  to  the  study 
of  mammalian  embryos.  For  reference,  the  methods  which  he  developed 
so  successfully  will  be  appended  without  material  change. 

Preparation  of  Material. — Pig  embryos,  10  mm.  or  more  in  length,  may  be  dissected 
easily,  mounted  as  opaque  objects,  and  used  for  several  years.  Success  in  dissecting 
such  small  embryos  depends;  (i)  on  the  fixation  and  hardening  of  the  material  employed; 
(2)  on  starting  the  dissection  with  a clean  cut  in  the  right  plane;  (3)  on  a knowledge  of  the 
anatomy  of  the  parts  to  be  dissected. 

Embryos  fixed  in  Zenker’s  fluid  have  given  the  best  results.  They  should  then  be  so 
hardened  in  95  per  cent  alcohol  that  the  more  diffuse  mesenchyme  will  separate  readily 
from  the  surfaces  of  the  various  organs,  yet  the  organs  must  not  be  so  brittle  that  they  will 
crumble  and  break.  Embryos  well  hardened  and  then  kept  for  two  weeks  in  80  per  cent 
alcohol  dissect  well.  Old  material  is  usually  too  brittle ; that  just  fixed  and  hardened  may 
prove  too  soft.  As  a test,  determine  whether  the  mesenchyme  separates  readily  from  the 
cervical  ganglia  and  their  roots. 

Dissecting  instruments  include  a binocular  dissecting  microscope,  a sharp  safety 
razor  blade,  large,  curved,  blunt-pointed  dissecting  needles,  pairs  of  small,  sharp-pointed 
forceps,  and  straight  dissecting  needles,  small  and  large. 

In  general,  it  is  best  to  begin  the  dissection  with  a clean,  smooth  cut  made  by  a single 
stroke  with  the  safety -razor  blade,  which  should  be  flooded  with  80  per  cent  alcohol.  The 
section  is  made  free  hand,  holding  the  embryo,  protected  by  a fold  of  absorbent  cotton, 
between  the  thumb  and  index  finger.  Having  made  preliminary  cuts  in  this  way,  the 
embryo  may  be  affixed  with  thin  celloidin  to  a cover  glass  and  immersed  in  a watch  glass 
containing  alcohol.  We  prefer  not  to  affix  the  embryo,  as  the  celloidin  used  for  this  pur- 
pose may  interfere  with  the  dissection.  Instead,  a cut  is  made  parallel  to  the  plane  of  the 
dissection  so  that  the  embryo,  resting  in  the  watch  glass  upon  this  flat  surface,  will  be  in  a 
fairly  stable  position.  It  may  thus  be  held  in  any  convenient  position  by  resting  the  con- 
vex surface  of  a curved,  blunt  dissecting  needle  upon  some  part  not  easily  injured.  The 
dissection  is  then  carried  on  under  the  binocular  microscope,  using  the  fine  pointed  forceps, 
dissecting  needles,  and  a small  pipette  to  wash  away  fragments  of  tissue. 

Whole  Embryos. — For  the  study  of  the  exterior,  whole  embryos  may  be  affixed  with 
celloidin  to  the  bottoms  of  watch  glasses  which  then  stack  in  wide-mouthed  jars  of  80 
per  cent  alcohol.  Such  specimens  last  several  years,  at  a saving  of  both  time  and  material. 
Preliminary  treatment  consists  in  immersion  in  9 5 per  cent  alcohol  one  hour,  in  ether  and 
absolute  alcohol  at  least  thirty  minutes,  in  thin  celloidin  one  hour  or  more.  Pour  enough 
thin  celloidin  into  a Syracuse  watch  glass  to  cover  its  bottom,  and  immerse  in  this  a circle 
of  black  mat  paper,  first  wet  with  ether  and  absolute  alcohol.  Pour  off  any  surplus  celloidin, 
mount  embryo  in  desired  position,  and  immerse  watch  glass  in  80  per  cent  alcohol,  in  which 
the  specimen  may  be  kept  indefinitely.  Embryos  may  also  be  mounted  in  gelatin-formalin 
solution  in  small,  sealed  glass  jars. 

Lateral  Dissections  of  the  Viscera. — Skill  is  required  to  demonstrate  most  of 
the  cranial  nerves,  but  the  central  nervous  system,  cranial  and  spinal  ganglia,  and  viscera 
may  easily  be  exposed.  Starting  dorsally,  make  a sagittal  section  of  the  embryo,  sHghtly 
to  one  side  of  the  median  line  and  avoiding  the  umbilical  cord  ventrally.  With  the  embryo 
resting  on  the  flat,  sectioned  surface,  begin  at  the  cervical  flexure,  and  with  fine  forceps 


4o6 


THE  STUDY  OF  PIG  EMBRYOS 


grasp  the  ectoderm  and  dural  anlage  at  its  cut  edge,  separate  it  from  the  neural  tube  and 
pia  mater,  and  strip  it  off  ventralwards,  exposing  the  myelencephalon  and  cervical  portion 
of  the  cord.  As  the  mesenchyme  is  pulled  away,  the  ganglia  and  roots  of  the  cranial  nerves 
will  be  exposed.  The  mesenchyme  between  the  ganglia  and  along  the  nerves  may  be 
removed  with  the  end  of  a small  blunt  needle.  Care  must  be  exercised  in  working  over  the 
mesencephalon  and  telencephalon  of  the  brain  not  to  injure  the  brain  wall,  which  may  be 
Vjrittle.  By  starting  with  a clean  dissection  dorsally,  and  gradually  working  ventrad,  the 
more  important  organs  may  l>e  laid  bare  without  injury.  The  beginner  should  compare 
his  specimen  with  the  dissections  figured,  and  also  previously  study  the  reconstructions. 

Median  Sagittal  Dissections. — Preliminary  to  the  dissection,  a cut  is  made  dorsally, 
as  near  as  possible  to  the  median  sagittal  plane.  Beginning  caudally,  at  the  mid-dorsal 
line,  an  incision  is  started  which  extends  in  depth  through  the  neural  tube  and  the  anlages 
of  the  vertebriE.  This  incision  is  carried  to  the  cervical  flexure,  cranial  to  which  point  the 
head  and  brain  are  halved  as  accurately  as  possible.  The  blade  is  then  carried  ventrally 
and  caudally,  cutting  through  the  heart  and  liver,  to  the  right  of  the  mid  plane  and  umbilical 
cord,  until  the  starting  point  is  reached.  A parasagittal  section  is  next  made,  well  to  the 
left  of  the  median  sagittal  plane,  and  the  sectioned  portion  is  removed,  leaving  on  the  left 
side  of  the  emlmyo  a plane  surface.  With  the  embryo  resting  upon  this  flat  surface,  the 
dissection  is  begun  by  removing  with  forceps  the  right  half  of  the  head.  In  pulling  this 
away  caudalwards,  half  of  the  dorsal  body  wall,  the  whole  of  the  lateral  body  wall,  and  the 
parts  of  the  heart  and  liver  lying  to  the  right  of  the  midplane  will  be  removed,  leaving  the 
other  structures  intact.  If  the  plane  of  section  were  accurate,  the  brain  and  spinal  cord 
will  be  halved  in  the  median  sagittal  plane.  Wash  out  the  cavities  of  the  brain  with  a 
pipette,  and  its  internal  structure  may  be  seen.  Dissect  away  the  mesenchyme  between 
the  esophagus  and  trachea  and  expose  the  lung.  Remove  the  right  mesonephros,  leaving 
the  proximal  part  of  its  duct  attached  to  the  urogenital  sinus.  The  right  dorsal  lobe  of  the 
liver  will  overlie  the  stomach  and  pancreas.  Pick  it  away  with  forceps  and  expose  these 
organs.  Dissect  away  the  caudal  portion  of  the  liver  until  the  hepatic  diverticulum  is  laid 
bare.  It  is  whitish  in  color  and  may  thus  be  distinguished  from  the  brownish  liver. 
Beginning  at  the  base  of  the  umbilical  cord,  carefully  pull  away  its  right  wall  with  forceps, 
thus  exposing  the  intestinal  loop  and  its  attachment  to  the  yolk  stalk.  If  the  umbilical 
artery  is  removed  in  the  caudal  portion  of  the  umbilical  cord,  the  allantoic  stalk  may  be 
dissected  out.  To  see  the  anlage  of  the  genital  gland,  break  through  and  remove  a part 
of  the  mesentery,  exposing  the  me,sial  surface  of  the  left  mesonephros  and  the  genital  fold. 
The  dissection  of  the  metanephros  and  ureter  is  difficult  in  small  embryos.  In  lo  to  12 
mm.  embryos,  the  umbilical  artery,  just  after  it  leaves  the  aorta,  passes  lateral  to  the 
metanephros  and  thus  locates  it.  By  working  carefully  with  fine  needles,  the  surface  of 
the  metanephros  may  be  laid  bare  and  the  delicate  ureter  may  be  traced  to  the  base  of  the 
mesonephric  duct.  The  extent  of  the  dorsal  aorta  may  also  be  seen  by  removing  the 
surrounding  mesenchyme.  With  a few  trials,  such  dissections  are  made  in  a short  time. 

Ventral  Dissections. — Ventral  dissections  of  the  viscera  are  very  easily  made.  With 
the  safety  razor  blade,  start  a cut  in  a coronal  plane  through  the  caudal  end  of  the  embryo 
and  the  lower  limb  buds.  Extend  this  cut  laterad  and  cephalad  through  the  body  wall  and 
the  upper  limb  bud.  The  head  may  be  cut  away  in  the  same  plane  of  section,  and  the 
cut  continued  through  the  body  wall  and  upper  limb  bud  of  the  opposite  side  back  caudally 
to  the  starting  point.  Section  the  embryo  in  a coronal  plane,  parallel  with  the  first  sec- 
tion and  near  the  back,  so  that  the  embryo  will  rest  upon  the  flattened  surface.  With 
forceps,  now  remove  the  ventral  body  wall.  By  tearing  open  the  wall  of  the  umbilical 
cord  along  one  side,  it  may  be  removed,  leaving  the  intestinal  loop  intact.  Pull  away  the 
heart,  noting  its  external  structure.  The  liver  may  also  be  removed,  leaving  the  stomach 


METHODS  FOR  THE  DISSECTION  OF  PIG  EMBRYOS 


407 


and  intestine  uninjured.  A portion  of  the  septum  transversum  covering  the  lungs  may  be 
carefully  stripped  away  and  the  lungs  thus  laid  bare. 

Dissections  made  in  this  way  show  the  trachea  and  lungs,  the  esophagus,  stomach  and 
dorsal  attachment  of  the  septum  transversum,  the  course  of  the  intestinal  canal,  and  also 
the  mesonephroi  and  their  ducts.  Favorable  sections  through  the  caudal  end  of  the  body 
may  show  the  urogenital  sinus,  rectum,  and  sections  of  the  umbilical  arteries  and  allantois. 
In  late  stages,  by  removing  the  digestive  organs,  the  urogenital  ducts  and  glands 
are  beautifully  demonstrated  (Figs.  147  and  148). 

Development  of  the  Face. — The  heads  of  pig  embryos  have  long  been  used  for 
the  study  of  the  development  of  the  face.  The  head  should  be  removed  by  passing  the 
razor  blade  between  the  heart  and  the  adjacent  surface  of  the  head,  thus  severing  the  neck. 
Next,  cut  away  the  dorsal  part  of  the  head  by  a section  parallel  to  the  ventral  surface,  the 
razor  blade  passing  dorsal  to  the  branchial  clefts  and  eyes.  Mount,  ventral  side  up,  three 
stages  from  embryos.  6,  12,  and  14  mm.  long,  as  shown  in  Figs.  369  and  67. 

Development  of  the  Palate. — This  may  be  studied  advantageously  in  pig  embryos  of 
two  stages;  (a)  20  to  25  mm.  long;  {h)  28  to  35  mm.  long.  Dissections  are  made  by  carry- 
ing a shallow  incision  from  the  anlage  of  the  mouth  back  to  the  external  ear  on  each  side 
(Fig.  73).  The  incisions  are  then  continued  through  the  neck  in  a plane  parallel  to  the  hard 
palate.  Before  mounting  the  preparation,  remove  the  top  of  the  head  by  a section  cutting 
through  the  eyes  and  nostrils,  parallel  to  the  first  plane  of  section.  Transverse  sections 
through  the  snout  may  also  be  prepared  to  show  the  positions  of  tongue  and  palatine  folds 
before  and  after  the  fusion  of  the  latter  (Fig.  72). 

Development  of  the  Tongue. — The  development  of  the  tongue  may  be  studied  from 
dissections  of  pig  embryos  6,  9,  and  13  mm.  long.  As  the  pharynx  is  bent  nearly  at  right 
angles,  it  is  necessary  to  cut  away  its  roof  by  two  pairs  of  sections  passing  in  different 


Fig.  419. — Diagram  to  illustrate  the  planes  of  section  made  in  dissecting  the  tongue. 

planes.  The  first  plane  of  section  cuts  through  the  eye  and  first  two  branchial  arches  just 
above  the  cervical  sinus  (Fig.  419, 1).  From  the  surface,  the  razor  blade  should  be  directed 
obliquely  dorsad  in  cutting  toward  the  median  line.  Cuts  in  this  plane  should  be  made 
from  either  side.  In  the  same  way,  make  sections  on  each  side  in  a plane  forming  an  obtuse 
angle  with  the  first  section  and  passing  dorsal  to  the  cervical  sinus  {II).  Now  sever  the 
remaining  portion  of  the  head  from  the  body  by  a transverse  section  in  a plane  parallel  to 
the  first  {III).  Place  the  ventral  portion  of  the  head  in  a watch  glass  of  alcohol,  and,  under 
the  dissecting  microscope,  remove  that  part  of  the  preparation  cranial  to  the  mandibular 
arches.  Looking  down  upon  the  floor  of  the  pharynx,  remove  any  portions  of  the  lateral 
pharyngeal  wall  which  may  still  interfere  with  a clear  view  of  the  pharyngeal  arches,  as 
seen  in  Fig.  84.  Permanent  mounts  of  the  three  stages  mentioned  above  may  be  made 
and  used  for  study  by  the  student. 


INDEX 


Abdomen,  muscles  of,  226 
Abdominal  pregnancy,  67 
Abducens  nerve,  282 
Abnormal  embryos,  76 
Accessorius  nerve,  285  ' 
Accessory  chromosome, ^29 
ganglia,  286 
genital  glands,  148 
pancreatic  duct,  113 
placentas,  67 
Acoustic  ganglion,  280 
meatus,  external,  310 
nerve,  280 
Acrania,  273 
Adipose  glands,  208 
tissue,  207 
Adolescence,  81 
After-birth,  67 
After-pains,  66 
Age  of  human  embryos,  68 
norms  of  human  embryos,  68 
Alse  nasi,  78 
Alar  plate,  248 
Albinism,  231 
Allantoic  stalk,  46 
vessels,  47,  54 
Allantois,  44 
birds,  46 
chick,  352 
mammals,  47 
man,  52 
reptiles,  46 
Allelomorphs,  28 
Alveoli  of  pancreas,  113 
of  lung,  1 1 8 

Alveolo-lingual  gland,  98 
groove,  96 
Ameloblasts,  91 
Amitosis,  3 
Amnion,  44 
birds,  44 
chick,  341 
mammals,  46 
man,  49 

anomalies  of,  51 
reptiles,  44 
Amniota,  10 
Amniotic  fluid,  51 
Amphiaster,  3 
Amphibia,  10 
cleavage  in,  31 
gastrulation  in,  35 
origin  of  mesoderm  in,  36 
of  notochord  in,  36,  42 
Ampulla  of  ductus  deferens,  157 
Ampulte,  356 
Amyelus,  273 


Anal  canal,  85 

membrane,  83,  145 
Anamniota,  10 
Anaphase  of  mitosis,  5 
Anchoring  villi,  57,  62 
Anencephaly,  273 
Angioblast,  169 
Animal  pole,  14 
Anlage,  8 

Ansa  hypoglossi,  278 
Antihelix,  312 
Antitragus,  312 
Anus,  85 

Aorta,  origin  of,  18 1 
I descending,  185 
dorsal,  187,  190 
ventral,  185 
Aortic  arches,  188,  189 

transformation  of,  189 
Apical  lung  bud,  1 16 
Appendages,  80 

anomalies  of,  81 
coronary,  126,  132 
Appendicular  skeleton,  221 
Appendix  epididymidis,  157 
testis,  157 
vermiform,  108 
Aqueduct,  cerebral,  260 
Archenteron,  34,  35 
Arches,  branchial,  77 
hyoid,  96,  229 
mandibular,  96 
pharyngo-palatine,  89 
vertebral,  214 
Archipallium,  267 
Arcuate  fibers,  257 
Area  opaca,  chick,  318 
parolfactory,  267 
pellucida,  chick,  318 
vasculosa,  169 
chick,  318 
Areolar  glands,  236 
tissue,  207 
Arm,  80 

Arrector  pili  muscle,  234 
Artemia,  5 

Arteries,  anomalies  of,  201 
axillary,  194 
basilar,  19 1 
brachial,  194 
carotid,  188,  189 
central,  of  retina,  302 
cerebral,  19 1 
chorioidal,  anterior,  19 1 
coeliac,  189,  193 
dorsal,  190 
epigastric,  192 

409 


4 -CO  INDEX 


Arteries,  femoral,  194 
gluteal,  194 
hepatic,  112 
hyaloid,  302 
hypogastric,  193 
iliac,  193,  194 
innominate,  189 
intercostal,  19 1 
interosseous,  194 
interscgmental , 1 86 
lateral,  192 
lumbar,  19 1 

mammary,  internal,  191 
median,  194 

mesenteric,  inferior,  189,  193 
superior,  187,  189,  193 
of  chick,  329,  340,  352 
of  extremities,  194 
of  leg,  194 

of  lower  extremity,  194 

of  pig,  362,  384 

of  u])per  extremity,  194 

ophthalmic,  191 

ovarian,  192 

peroneal,  194 

phrenic,  192 

popliteal,  194 

pulmonary,  118,  18 1,  190 

radial,  194 

renal,  192 

sacral,  middle,  193 

sciatic,  194 

spermatic,  192 

spinal,  190 

stapedial,  310 

subclavian,  188,  189,  19 1,  194 

suprarenal,  192 

ulnar,  194 

umbilical,  185,  193 

ventral,  192 

vertebral,  190 

vitelline,  186 

Artificial  parthenogenesis,  26 
Arytenoid  cartilage,  116,  221 
ridges,  96 
swellings,  115 
Ascaris,  maturation  in,  1 7 
megalocephala  univalens,  5 
Ascending  colon,  108 
Atlas,  215 
Atrial  canal,  175 
septa,  176 

Atrio-ventricular  bundle,  184 
canal,  1 76 
valves,  183 
Atrium,  174,  176 
Auditory  ossicles,  310 
placode,  303 
tube,  100,  310 
vesicle,  305 
Auricle  of  ear,  3 1 1 
Auricular  fold,  31 1 
Axial  skeleton,  212 
Axillary  artery,  194 
nerve,  278 
vein,  200 

Axis  (epistropheus),  215 
cylinder,  239 


Axon,  239 
Azygos  vein,  199 

Baby,  blue,  183 
Back,  muscles  of,  226 
Bars,  sternal,  216 
Bartholin’s  gland,  149 
Basal  plate,  63,  248 
Basilar  artery,  19 1 
membrane,  309 
Basilic  vein,  200 
Basophils,  173 
Bertin’s  renal  columns,  141 
Bicuspid  valve,  184 
Bile  ca])illaries,  1 12 
duct,  common,  no 
Biogenesis,  law  of,  9 
Birds,  10 

cleavage  in,  32 
gastrulation  in,  33 
origin  of  mesoderm  in,  36 
of  notochord  in,  39 
Birth,  66,  81 
Birthmarks,  231 
Bladder,  147 

anomalies  of,  148 
trigone  of,  147 

Blastemal  stage  of  skeleton,  212 
Blastocoele,  30 
1'  Blastoderm,  33 
Blastodermic  vesicle,  34 
Blastomere,  30 
Blastopore,  34,  33 
Blastula,  31 
of  chick,  314 
Blood  cells,  169 
ichthyoid,  171 
origin  of,  169 
sauroid,  1 7 1 
corpuscles,  red,  171 

white,  171.  See  also  Leucocytes. 
elements,  monophyletic  theory,  169 
polyphyletic  theory,  169 
islands,  169 
lacunae,  37 
Blood  platelets,  173 
vessels,  183 

anomalies  of,  201 
changes  at  birth,  202 
chick,  321,  329,  339,  332 
origin  of,  169 
pig,  361,  3«4 
primitive,  183 
Blue  baby,  183 
Body  cavities,  8 
aortic,  289 
' carotid,  290 

I chromaffin,  289 

I ciliary,  304 

geniculate,  260 
' mammillary,  261 

of  vertebra,  214 
pineal,  260 
plan,  10 

postbranchial,  102 
segmentation  of,  7 
ultimobranchial,  102 
vitreous,  302 


INDEX 


411 


Body  stalk,  47,  50,  52 
Bone,  208 
atlas,  215 
axis,  215 
carpal,  221 
cartilage,  208,  210 
cells,  209 
clavicle,  221 
compact,  209 
coxal,  222 
destroyers,  209 
development,  cartilage,  210 
endochondral,  210 
membrane,  208 
perichondral,  210 
periosteal,  210 
ear,  221 
ethmoid,  218 
femur,  222 
fibula,  222 
formers,  208 
frontal,  220 
growth  of,  210 
histogenesis  of,  208 
humerus,  221 
hyoid,  221 
ilium,  222 
ischium,  222 
lacrimal,  220 
lacunae,  209 
mandible,  220 
marrow,  210 
red,  209 
yellow,  210 
maxillary,  220 
membrane,  208 
of  skull,  219 
metacarpal,  222 
metatarsal,  222 
nasal,  220 
occipital,  218 
palate,  220 
parietal,  220 
patella,  222 
phalanges,  222 
pisiform,  222 
pubis,  222 
radius,  221 
ribs,  215 
sacrum,  216 
scapula,  221 
sesamoid,  212 
sphenoid,  218 
sternum,  216 
tables  of,  209 
tarsal,  222 
temporal,  219 
tibia,  222 
ulna,  221 
vomer,  220 
zygomatic,  220 
Border  vein,  200 
Bowman’s  capsule,  138,  142 
Brachial  artery,  194 
plexus,  278 
vein,  200 

Brachium  conjuctivum,  259 
pontis,  258 


Brain,  251 

anomalies  of,  273 
cavities  of,  252 
chick,  328,  337,  350 
divisions  of,  251 
flexures  of,  253 
olfactory,  264 
pig.,  355.  378 
vesicles,  primary,  246 
Branchial  arches,  77 
chick,  338,  350 
derivatives  of,  220 
pig.  354 
clefts,  77 

anomalies  of,  77 
cysts,  77 
duct,  100 
fistulas,  77 
groove,  99 
pouches,  99 
Branchiomerism,  229 
Bridge  of  nose,  78 
Broad  ligament,  160 
Bronchi,  infracardiac,  117 
ventral,  116 
Bronchial  buds,  116 
Brunner,  duodenal  glands  of,  107 
Bulb,  hair,  234 
olfactory,  266 
Bulbar  swellings,  181 
Bulbo-urethral  gland  (ofj'Cowper),  148 
Bulbus  cordis,  174 
Bundle,  atrio-ventricular,  184 
ground,  250 

median  longitudinal,  257 
Bursa  infracardiaca,  12 1 
omental,  12 1 

inferior  recess  of,  12 1 
pharyngeal,  100 


Calcar  avis,  272 
Calcarine  fissure,  272 
Calyces  of  metanephros,  141 
Canal,  anal,  85 
atrial,  175 

atrio-ventricular,  1 76 
central,  248,  253 
digestive,  103 
hyaloid,  303 
incisive  (of  Stenson),  89 
inguinal,  160 
neurenteric,  38,  41 
notochordal,  40 
pleuro-pericardial,  128 
pleuro-peritoneal,  128 
Stenson 's,  89 
Canaliculi,  dental,  92 
Canalized  fibrin,  64 
Capillaries,  bile,  112 
Capsule,  Bowman’s,  138,  142 
cells,  of  ganglia,  244 
internal,  265 
of  lens,  302 

of  liver  (Glisson’s),  109 
periotic,  217 
Cardiac  muscle,  223 
stomach,  104 


412 


INDEX 


Cardinal  vein,  187 

anterior,  187,  197 
common,  187,  197 
posterior,  187,  199 
pro-,  187,  197 
sub-,  199 
su])ra-,  199 

Carotid  artery,  common,  189 
external,  189 
internal,  188,  189 
body,  290 
Carpus,  221 
Cartilage,  208 

arytenoid,  116,  221 
bone,  210,  268 

development  of,  210 
corniculate,  116,  221 
cricoid,  116,  221 
cuneiform,  1 16,  221 
differentiation  of,  208 
elastic,  208 
fibro-,  208 
growth  of,  208 
hyaline,  208 
Meckel’s,  220 
of  larynx,  1 16 
Reichert’s,  ,^io 
thyroid,  1 16,  221 
Cauda  equina,  251 
Caudate  lobe  of  liver,  126 
nucleus,  265 
Caul,  51 

Caval  mesentery,  121,  200 
Cavity,  body,  8,  37,  127 
brain,  252 
head,  229 
joint,  21 1 
marrow,  210 
medullary,  210 
mouth,  85 

of  mesodermal  segment,  128 
oral,  85 

pericardial,  127,  133 
peritoneal,  128,  133 
pleural,  128,  129,  133 
pleuro-pericardial,  128 
pleuro-peritoneal,  127 
synovial,  21 1 
tympanic,  100,  310 
Cecum,  106 
Cell-chain  theory,  245 
Cells,  blood,  169 
bone,  209 

capsule,  of  ganglia,  244 
chromaffin,  289 
cone,  301 
decidual,  61,  64 
division  of,  3 
equational,  19 
reductional,  18 
enamel,  90 

ependymal,  239,  242, "^248 
ethmoidal,  218 
follicle,  14,  20 
ganglion,  241 
germ,  12,  154 
germinal  nerve,  239 
giant,  173,  209 


Cells,  Hofbauer,  62 

interstitial,  of  testis,  154 
mass,  inner,  33 
intermediate,  8 
mastoid,  219 
mitral,  279 
multiplication  of,  3 
muscle,  223 
nerve,  237 

neuroglia,  237,  239,  242 
primitive  blood,  169,  170 
primordial  germ,  16,  149 
Purkinje,  258 
pyramidal,  273 
rod,  301 
sense,  292 
sheath,  244 
sperm,  14 
supporting,  293 
of  neural  tube,  242 
of  spinal  ganglia,  241 
sustentacular  (of  Sertoli),  16,  17,  154 
taste,  293 

Cement,  of  teeth,  93 
Centers  of  ossification,  21 1 
Central  artery,  302 
canal,  248,  253 
nervous  system,  245 

chick,  327,  337,  350 
pig-  355.  378 
sulcus,  292 
Centrosome,  3,  12 
Cephalic  flexure,  25 
vein,  200 

Cerebellar  hemisphere,  258 
Cerebellum,  251,  258 
Cerebral  aqueduct,  253,  260 
artery,  anterior,  191 
middle,  19 1 
piosterior,  19 1 
cortex,  histogenesis  of,  273 
hemispheres,  270 
peduncles,  260 
sulci,  273 
veins,  197 
Cervical  duct,  100 
enlargement,  250 
flexure,  254 
mucous  plug,  61 
plexus,  278 
sinus,  77 

sympathetic  ganglia,  289 
vesicle,  100 
Cervix  of  uterus,  1 58 
Chamber,  anterior,  of  eye,  304 
Changes,  growth,  81 
in  form,  82 
Cheek,  87 

Chick  embryos,  313 
of  first  day,  314 

transverse  sections,  315 
of  five  segments  (twenty-three  hours),  318 
longitudinal  section,  319 
of  seven  segments  (twenty-five  hours),  319 
transverse  sections,  322 
of  seventeen  segments  (thirty-eight 
hours),  326 

transverse  sections,  331 


INDEX 


413 


Chick  embryos,  of  three  to  four  days,  349 
unincubated  ovum,  313 
Chin,  78 

Choanae,  primitive,  87,  294 
secondary,  87,  295 
Chondrification  of  skull,  217 
of  vertebrae,  214 
Chondrocranium,  217 
ossification  of,  218 
Chorda  dorsalis,  42,  212,  216 
gubernaculi,  160,  161 
tympani,  285 
Chordinae  tendineae,  183 
Chorioid  fissure,  of  brain,  267,  271 
of  eye,  299 
layer  of  eye,  304 
plexus,  257,  260,  267 
Chorioidal  artery,  anterior,  191 
Chorion,  44 

frondosum,  58,  62 
in  birds,  44 
in  chick,  341 
in  mammals,  46 
in  reptiles,  44 
laeve,  58,  62 
villi  of,  49,  54,  57,  62 
Chorionic  circulation,  185 
Chromaffin  bodies,  289 
aortic,  289 
cells,  289 

Chromatin  network,  12 
Chromosomes,  4 
accessory,  29 
conjugation  of,  28 
number  of,  5 
sex,  29 
X-,  29 
Y-,  29 
Cilia,  304 
Ciliary  bodies,  304 
muscle,  304 

Circulation,  changes  at  birth,  202 
chorionic,  185 
fetal,  201 
intervillous,  66 
vitelline,  186 
Circulatory  system,  185 
Cisterna,  chyli,  203 
Clava,  258 
Clavicle,  221 
Cleavage  of  ovum,  30 
amphibia,  31 
Amphioxus,  30 
birds,  32 
chick,  314 
higher  fishes,  32 
lower  fishes,  31 
mammals,  33 
primates,  34 
reptiles,  32 
Cleft  palate,  89 
sternum,  222 
xiphoid  process,  222 
Clefts,  branchial,  77 
anomalies  of,  77 
Clitoris,  165 
Cloaca,  85 

anomalies  of,  148 


Cloaca,  differentiation  of,  145 
pig.  360,  383 

Cloacal  membrane,  85,  145 
Closing  plates,  99 
ring,  63 

Coccygeal  body,  205 
Cochlear  duct,  306 
Coeliac  artery,  188,  193 
axis,  primitive,  193 
Ccelom,  8,  37,  127 
chick,  320 
pig.  360,  383 
primitive,  127 
Coelomic  pouches,  36 
Coitus,  24 
Colic  valve,  108 
Collateral  eminence,  272 
fissure,  272 
ganglia,  287 
Collecting  tubules,  139 
Colliculus,  facial,  285 
inferior,  260 
seminalis,  159 
superior,  260 
Coloboma,  305 
Colon,  108 
Column,  gray,  248 
muscle,  224 
of  fornix,  268 
renal,  141 

Commissure,  anterior,  268 
gray,  249 
hippocampal,  268 
of  telencephalon,  268 
posterior,  166 
white,  249 

Communicating  rami,  287 
Compact  bone,  209 
layer,  of  uterus,  61,  62 
Comparison  of  ovum  with  spermatozoon,  15 
Concealed  testis,  163 
Conceptions,  fundamental,  8 
Conchse,  219,  297 
Concrescence,  theory  of,  39 
Cone  cells,  301 
Conjoined  twins,  42 
Conjunctiva,  304 
Connective  tissue,  206 
Continuity  of  germ  plasm,  9 
Convoluted  tubules,  142 
Convolutions  of  hemispheres,  271 
Copula,  96 
Copulation,  24 
Coracoid  process,  221 
Cord,  genital,  130 

intermediate,  153,  154 
liver,  1 1 2 

nephrogenic,  137,  140 
spermatic,  163 
spinal,  246 
testis,  153 
umbilical,  54 
vocal,  1 16 
Corium,  231 
Cornea,  304 

Corniculate  cartilages,  116,  221 
Cornification,  231,  234 
Corona  radiata,  23 


4M 

Coronary  ai>pendagcs,  126,  132 
ligament,  126 
sinus,  179,  197 
sulcus,  175 

Corpora  cavernosa  penis,  165 
quadrigemina,  260 
Corjuis  albicans,  23 
callosum,  268,  269 
cavcrnosum  urethra?,  165 
hcmorrhagicum,  23 
luteum,  23 
spurium,  23 
verum,  23 
striatum,  264 

Corpuscles,  blood,  red,  170 

white,  I 71.  See  a\so  Lencocyles. 
lamellated,  292 
renal,  139,  142 
siilenic,  205 
tactile,  292 
thymic,  loi 

Cortex,  cerebral,  266,  267,  273 
histogenesis  of,  273 
of  hair,  234 
of  kidney,  14 1 
Corti’s  organ,  307 
jjillars,  307 
Costal  processes,  214 
Costo-cervical  trunk,  191 
Cotyledons  of  placenta,  65 
Cowper’s  gland,  148 
Coxal  bone,  222 
Cranial  nerves,  278 
Cranioschisis,  273 
Crest,  ganglion,  241 
inguinal,  160 
neural,  241 

Cribriform  plates,  219 
Cricoid  cartilage,  i 16,  22  r 
Crista  ampullaris,  306,  307 
galli,  218 
terminalis,  179 
Crossing-over,  28 
Crown-lieel  length,  68,  69 
Crown-rump  length,  68,  69 
Crus  longum,  310 
Cryptorchism,  163 
Cumulus  oophorus,  20 
Cuneiform  cartilage,  116,  221 
Cuncus,  258 
Cup,  optic,  297,  298 
Curvature  of  stomach,  greater,  104 
lesser,  104 
Cuticle  of  hair,  234 
Cutis  plate,  231 
Cuvier’s  ducts,  187,  197 
Cyclopia,  305 
Cystic  duct,  no 
kidney,  145 

Cysts,  dermoid,  156,  231 
Cytomorphosis,  6 
Cytoplasm,  12 
Cytoplasmic  inheritance,  29 

Darwin’s  tubercle,  312 
DecidujE,  60 
basalis,  60,  62 
capsularis,  60,  62 


INDEX 

Decidua?,  vera,  60,  61 
Decidual  cells,  61,  64 
membranes,  58 
separation  of,  66 
teeth,  90 

periods  of  eruption  of,  93 
Delamination,  8 
Delivery,  date  of,  69 
Dendrites,  239 
Dendrons,  239 
Dental  canaliculi,  92 
lamina,  89 
papilla,  90,  92 
pulp,  92 
ridge,  89 
sac,  93 

Dentate  gyrus,  267 
nucleus,  259 

Dentinal  fibers  (of  Tomes),  92 
Dentine,  92 

Derivatives  of  germ  layers,  6 
Derma,  231 
Dermatome,  231 
Dermoid  cyst,  156,  231 
Descending  aorta,  185 
colon,  108,  184 
tract  of  fifth  nerve,  258,  283 
Descent  of  heart,  184 
of  ovary,  161 
of  testis,  1 61 
Determination  of  sex,  29 
Deutoplasm,  12 

Development,  general  features'of,  3 
postnatal,  8i 
prenatal,  69,  81 
Developmental  period,  1,81 
processes,  8 
Dextrocardia,  184 
Diaphragm,  134 
anomalies  of,  134 
dorsal  pillars  of,  132 
Diajjhragmatic  hernia,  134 
ligament,  160 
Diaphysis,  21 1 
Diaster,  5 

Diencephalon,  251,  260 
Digestive  glands,  97,  109,  112 
system,  85 

chick,  337,  351 
pig,  357,  379 
tube,  103 

Digits,  supernumerary,  81 
Diploe,  209 
Disc,  germinal,  33 
intercalated,  224 
intervertebral,  214 
Dissecting  instruments,  405 
Dissection  of  pig  embryos,  406 
Diverticulum,  hepatic,  109 
Meckel's,  52,  109 
Nuck’s,  163 
of  ileum,  32,  109 
of  pharyngeal  pouches,  99 
Dominance,  28 
Dorsal,  aorta,  187,  190 

mesentery,  differentiation  of,  120 
mesocardium,  126,  173 
mesogastrium,  12 1 


INDEX 


415 


Dorsal,  motor  nucleus,  285 
pancreas,  112 
pillars  of  diaphragm,  132 
Double  monsters,  42 
Duct,  accessory  pancreatic,  113 
branchial,  100 
cervical,  100 
cochlear,  306 
common  bile,  no 
Cuvier’s,  187,  197 
cystic,  1 10 
Ebner's,  99 
ejaculatory,  157 
endolympir,  306 
Gartner’s  (of  epoophoron),  157 
hepatic,  no 
lacrimal,  305 
mesonephric,  139,  150 
transformation  of,  156 
milk,  236 
Mullerian,  150 

transformation  of,  157 
naso-lacrimal,  305 
pancreatic,  113 
papillary,  141 

para-urethral  (of  Skene),  148 
primitive  genital,  150 
pronephric  (primary  excretory),  135,  137 
semicircular,  306 
thoracic,  203 
thyroglossal,  103 
Wolffian,  139 
Ductuli  abberantes,  156 
efferentes,  156 
Ductus  arteriosus,  189,  203 
choledochus,  no 
deferens,  157 
endolymphaticus,  306 
epididymidis,  156 
reunions,  306 
venosus,  197,  203 

Duodenal  glands  (of  Brunner),  107 
Duodenum,  106 
Dwarfs,  84 
Dyads,  17 

Ear,  305 

auricle  of,  311 
external,  80,  305,  310 
internal,  305 
middle,  303,  310 
Ebner’s  gland,  97 
Ectoderm,  5,  34 
formation  of,  34 
Ectodermal  derivatives,'6,^230 
Ectopic  pregnancy,  67 
Ectoplasm,  206 
Efferent  ductules,  156 
Ejaculatory  duct,  157 
Elastic  cartilage,  208 
tissue,  207 
Eleidin,  231 

Ellipsoids  of  spleen,  205 
Embryology,  definition  of,  i 
history  of,  2 
scope  of,  I 
value  of,  2 
Embryos,  chick,  313 


Embryos,  chick,  of  first  day,  314 
transverse  sections,  315 
of  five  segments  (twenty-three  hours),  318 
longitudinal  section,  319 
of  seven  segments  (twenty-five  hours), 

319 

transverse  sections,  322 
of  seventeen  segments  (thirty-eight 
hours),  326 

transverse  sections,  331 
of  three  to  four  days,  349 
unincubated  ovum,  313 
human,  69 
age  of,  68 
age-norms,  68 
anomalies  of,  76 
crown-heel  length,  68 
crown-rump  length,  68 
estimation  of  age,  68,  69 
of  Arey,  75 

of  Bardeen  and  Lewis,  227 
of  Bryce-Teacher,  69 
of  Coste,  72 
of  Eternod,  71 
of  fifth  week,  71 
of  fourth  week,  71 
of  His,  2.6  mm.,  72 
4.2  mm.,  73 
of  Ingalls,  70 
of  Kollman,  246 
of  Kromer,  71 
of  Mall,  2.0  mm.,  53 
7.0  mm.,  74 
of  Mateer,  70 
of  Miller,  69 
of  Peter,  69 
of  Prentiss,  74 
of  second  week,  69 
of  six  to  eight  weeks,’’73 
of  Spee,  70 
of  third  week,  70 
period  of,  69 
weight  of,  68 
pig-  353 

dissection  methods,  405 
early  stages,  353 
eighteen  mm.  stage,  400 
six  mm.  stage,  354 

transverse  sections,  367 
ten  to  twelve  mm.  stage,  376 
transverse  sections,  387 
thirty-five  mm.  stage,  402 
Embryotroph,  57 
Eminence,  collateral,  272 
Enamel  cells,  90 
layer,  91 
organ,  89 
pulp,  90 

Encephalocoele,  273 
Endocardial  cushions,  176,  183 
Endocardium,  173 
Endochondral  ossification,  210 
Endolymph  duct,  306 
sac,  306 

Endoplasm,  206 
Endothelium,  169 
Enlargement,  cervical,  250 
lumbar,  250 


4i6 

Entoderm,  5,  _-:(4 

formation  of,  34,  35,  36 
Entodermal  derivatives,  6 
Eosinophils,  173 
Eparterial  lunfj;  bud,  116 
Ependymal  cells,  239,  242,  248 
layer,  246 
zone,  238 
Epicardium,  173 
Epidermis,  230 
anomalies  of,  231 
Epididymis,  156 
appendix  of,  157 
efferent  ductules  of,  156 
Epigastric  artery,  192 
Epigenesis,  2 
Epiglottis,  96,  1 15,  1 16 
Epiphyseal  line,  21 1 
Epiphysis  (pineal  body),  260 
of  bone,  21 1 

Epiploic  foramen  (of  Winslow),  121 
Epispadias,  168 
Epistropheus,  215 
Epithalamus,  260 
Epithelial  bed,  234 
sheath,  91 
Epithelium,  5 
germinal,  156 
lens,  301 
olfactory,  295 
respiratory,  117,  296 
sensory,  237 
Epitrichium,  231 
Eponychium,  232 
Epoophoron,  157 
ducts  of,  157 
Equational  mitosis,  19 
Eruption  of  teeth,  93 
Erythroblasts,  171 
Erythrocytes,  170 
origin  of,  170 
Erythroplastids,  170 
Esophagus,  103 
anomalies  of,  104 
pig,  35S,  382 

Establishment  of  external  form,  76 
Ethmoid  bone,  218 
Ethmoidal  cells,  218,  297 
Ethmo-turbinals,  297 
Eustachian  tube,  100 
valve,  179 
Evaginations,  8 

Excretory  duct,  primary,  135,  137 
Expression,  muscles  of,  229 
External  ear,  80,  310 

form,  establishment  of,  76 
genitalia,  163 

Extra-embryonic  mesoderm,  42 
structures,  44 

Extra-uterine  pregnancy,  67 
Extremities,  arteries  of,  194 
muscles  of , 227 
veins,  of,  200 
Eye,  79,  297 

anomalies  of,  305 
chick,  328,  337,  350 
pig.  357.  379 

Eyeball,  muscles  of,  228,  229 


INDEX 

Eyelashes,  304 
Eyelids,  304 

Face,  77 

anomalies  of,  79 
Facial  colliculus,  285 
nerve,  284 

Falciform  ligament,  126 
False  hermaphroditism,  168 
Fascia,  207 
Fasciculi  proprii,  250 
Fasciculus  cuneatus,  250 
gracilis,  250 
Fecal  fistula,  32 
Female  urethra,  147 
Femoral  artery,  194 
nerve,  278 
vein,  200 
Femur,  222 
Fertilization,  25 
in  chick,  313 
in  man,  27 
I in  mouse,  26 
in  Tarsius,  27 
results  of,  26 
significance  of,  26 
time  of,  27 
Fertilizin,  26 
Fetal  circulation,  201 
membranes,  44 
birds,  44 
mammals,  46 
man,  49 
reptiles,  44 
placenta,  62 
Fetus,  73,  81 
period  of,  75 
Fibrils,  keratin,  232 
Fibrin,  canalized,  64 
Fibro-cartilage,  208 
I Fibula,  222 

Filament,  axial,  of  spermatozoon, 
spiral,  of  spermatozoon,  15 
j terminal,  of  spermatozoon,  15 
[ Filiform  papillas,  96 
Filum  terminale,  251 
Fishes,  10 

higher,  cleavage  in,  31 
lower,  cleavage  in,  32 
Fissure,  271 
calcarine,  272 
chorioid,  267,  271,  299 
collateral,  272 
hippocampal,  271 
lateral,  272 
longitudinal,  266 
median,  248,  250 
of  Rolando,  272 
parieto-occipital,  272 
rhinal,  271 
Sylvian,  272 
Fistula,  fecal,  32 
Flagellum  of  spermatozoon,  15 
Flexure,  233 
cephalic,  253 
cervical,  234 
chick,  336 
pig.  354 


INDEX 


417 


Flexure,  pontine,  254 
Flocculus,  259 
Floor  plate,  246 
Folds,  8 

genital,  137,  149 
head,  71 
chick,  315 
inguinal,  158,  160 
mesonephric,  137 
nail,  231 
neural,  237 
tail,  71 
urethral,  163 
urogenital,  135,  137,  149 
Foliate  papillse,  97 
Follicle  cells.  14,  20 
primordial,  20,  156 
rupture  of,  23 
thyroid,  103 

vesicular  (Graafian),  20,  23,  156 
Fontanelle,  220 
Foramen,  cecum,  96,  103 
epiploic  (of  Winslow),  12 1 
incisive,  89 
interatrial,  176 

interventricular,  of  heart,  183 
closure  of,  183 
of  brain,  266 
mandibular,  221 
Monro’s,  266 
ovale,  176,  181 
closure  of,  203 
transverse,  215 
Winslow’s,  121 
Fore-brain,  251 
Fore-gut,  85 

chick,  318,  320,  329 
Forehead,  78 
Fore-skin,  165 
Form,  changes  in,  82 
Fornix,  268 

columns  of,  268 
Fossa,  olfactory,  294 
oral,  85 
ovalis,  181 
supratonsillar,  100 
tonsillar,  100 
Free  chorionic  villi,  62 
nerve  terminations,  292 
Frenulum  prepucii,  165 
Frontal  bone,  220 
lobe,  270 
operculum,  272 
sinus,  297 

Fronto-nasal  process,  77,  294 
Froriep’s  ganglion,  282 
Functional  classification  of  nerves,  275 
Fundamental  conceptions,  8 
processes,  in  myogenesis,  225 
Fundus,  of  uterus,  158 
Fungiform  papillae,  96 
Funiculi,  of  spinal  cord,  250 
Furcula,  of  His,  115 

Gall  bladder,  1 10 
Ganglion,  274 
accessory,  286 
acoustic,  280 
27 


Ganglion,  cell  layer,  of  retina,  279,  301 
cells,  241 
collateral,  287 
crest,  241 
Froriep’s,  282 
geniculate,  284 
jugular,  286 
nodose,  286 
otic,  280 
petrosal,  285 
semilunar,  283 
spinal,  241 
spiral,  280 
superior,  285 

supporting  elements  of,  244 
sympathetic,  242,  287 
terminal,  288 
vagus  accessory,  286 
vestibular,  280 
Gartner’s  ducts,  157 
Gastric  glands,  105 
Gastro-colic  ligament,  124 
Gastro-splenic  ligament,  124 
Gastrula,  34 
i Gastrulation,  34 
amphibia,  35 
Amphioxus,  34 
birds,  35 
chick,  314 
mammals,  36 
, reptiles,  35 

General  features  of  development,  3 
Genes,  28 

Genetic  restriction,  larv  of,  9 
Geniculate  bodies,  260 
ganglion,  284 
Genital  cord,  150 
ducts,  150 
fold,  137,  149 
of  pig,  360,  383 
glands,  149,  153,  154 
accessory,  148 
organs,  149 
tubercle,  163 
Genitalia,  external,  163 
anomalies  of,  168 
internal,  153 

ligaments  of,  159 
Genu,  285 
Germ  cells,  12,  154 

primordial,  16,  149 
layers,  5 

derivatives  of,  6 
origin  of,  34 
plasm,  continuity  of,  9 
Germinal  disc,  33 
epithelium,  136 
nerve  cells,  239 
Gestation,  period  of,  69 
Giant  cells,  173,  209 
Giants,  84 
Gill  slits,  77 

Glands,  accessory  genital,  148 
adipose,  208 
alveolo-lingual,  98 
areolar,  236 
Bartholin’s,  149 
Brunner’s,  107 


INDEX 


418 

Glands,  bulbo-urcthral,  148 
carotid,  290 
coccygeal,  205 
Cow])cr’s,  148 
digestive,  97,  109,  112 
duodenal  (of  Brunner),  107 
Ebner’s,  97 
gastric,  105 
genital,  149,  153,  154 
hemal,  204 
hemolymph,  204 
intestinal,  107 
lacrimal,  304 
lymph,  204 
mammary,  235 
rudimentary,  236 
supernumerary,  236 
Meibomian,  304 
Moll’s,  304 
Montgomery’s,  236 
of  jjregnancy,  61 
parathyroid,  loi 
parotid,  97 
pineal,  260 
prostate,  148 
salivary,  97 
sebaceous,  234 
sublingual,  98 
submaxillary,  98 
sudoriferous,  234 
suprarenal,  290 
sweat,  234 
tarsal,  304 
thymus,  100 
thyroid,  103 
urinary,  135 

uterine,  of  pregnancy,  61 
vestibular,  149 
Zeiss’,  304 
Gians  clitoridis,  165 
penis,  165 

Glisson’s  capsule,  109 
Glomerulus,  135,  138,  142,  279 
Glomus  caroticum,  290 
coccygeum,  205 
Glossopharyngeal  nerve,  283 
Glottis,  96,  1 15 
Gluteal  artery,  194 
vein,  200 
Gonads,  149 

Graafian  follicle,  20,  22,  156 
Granular  layer  of  cerebellum,  259 
leucocytes,  172 
Granules,  pigment,  231 
Gray  column,  248 
commissures,  249 
rami,  287 

Greater  curvature  of  stomach,  104 
omentum,  104,  12 1 
Groove,  alveolo-lingual,  96 
branchial,  99 
labial,  89 

laryngo-tracheal,  T14 
naso-lacrimal,  305 
neural,  237 
chick,  316 
])rimitive,  39,  40 
rhoml)ic,  257 


Groove,  urethral,  163 
Ground  bundles,  250 
Growth  changes,  8i 
anomalies  of,  84 
in  length,  82 
in  weight,  82 
of  organ  systems,  82 
of  organs,  82 

Gubernaculum  testis,  161,  162 
Gustatory  organ,  292 
Gyrus,  273 
dentate,  267 
hippocampal,  267 

Hair,  232 

anomalies  of,  234 
bulb,  234 
cortex,  234 
cuticle,  234 
epithelial  bed,  234 
lanugo,  234 
medulla,  234 
papilla,  233 
shaft,  234 
sheath,  234 
Hard  palate,  89 
j Hare  lip,  79 
; Haversian  system,  210 
j Head,  76 
chick,  318 
cavities,  229 
j fold,  71 

chick,  315 

I of  spermatozoon,  1 5 
j muscles  of,  228 

I process,  39,  40,  42 

j chick,  3 1 5 

segmentation  of,  229 
vein,  primary,  197 
I Heart,  173 
I anomalies  of,  184 
I chick,  321,  329 

I descent  of,  184 

; differentiation  of  wall  of,  184 

i pig,  361,  384 

: Helix,  312 
hyoid,  31 1 
Hemal  gland,  204 
Hemiazygos  vein,  199 
Hemicrania,  273 
Hemispheres,  cerebral,  251,  270 
cerebellar,  258 
Hemoblast,  170 
Hemolymph  gland,  204 
Hemopoiesis,  169 
Hemotrophic  nutrition,  57 
Henle’s  loop,  142 
Hensen’s  knot,  39 
Hepatic  artery,  112 
diverticulum,  109 
duct,  1 10 

sinusoids,  iio,  194 
I vein,  196 

common,  200 

j Hepato-duodenal  ligament,  126 
! Hepato-gastric  ligament,  126 
^ Heredity,  Mendel’s  law  of,  28 
' Hermaphroditism,  168 


INDEX 


419 


Hermaphroditism,  false,  168 
Hernia,  diaphragmatic,  134 
inguinal,  163 
umbilical,  107,  109 
Hind -brain,  251 
Hind-gut,  85 
chick,  338,  360 
Hip  bone,  272 

Hippocampal  commissure,  268 
fissure,  271 
gyrus,  267 

Hippocampus,  267,  272 
His,  furcula  of,  115 
Histogenesis,  6 
of  bone,  208 
of  blood,  168 
of  cartilage,  208 
of  connective  tissue,  206 
of  muscle,  223 
of  nervous  tissue,  237 
Historical,  2 
Hofbauer  cells,  62 
Holoblastic  ovum,  30 

Homologues  of  mesoderm  and  notochord,  42 
Horn,  greater,  of  hyoid  bone,  221 
lesser,  of  hyoid  bone,  221 
Horse-shoe  kidney,  144 
Human  embryos,  69 
age  of,  68 
age-norms,  68 
anomalies  of,  76 
crown-heel  length,  68 
crown-rump  length,  68 
estimation  of  age,  68,  69 
of  Arey,  75 

of  Bardeen  and  Lewis,  227 
of  Bryce-Teacher,  69 
of  Coste,  72 
of  Eternod,  71 
of  fifth  week,  71 
of  fourth  week,  71 
of  His,  2.6  mm.,  72 
4.2  mm.,  73 
of  Ingalls,  70 
of  Kollman,  246 
of  Kromer,  71 
of  Mall,  2.0  mm.,  53 
7.0  mm.,  74 
of  Mateer,  70 
of  Miller,  69 
of  Peter,  69 
of  Prentiss,  74 
of  second  week,  69 
of  six  to  eight  weeks,  73 
of  Spee,  70 
of  third  week,  70 
period  of,  69 
weight  of,  68 
Humerus,  221 
Hyaline  cartilage,  208 
Hyaloid  artery,  302 
canal,  303 
Hydramnios,  51 
Hydrocephaly,  273 
Hymen,  157,  158 
anomalies  of,  159 
Hyoid  arch,  96,  229 
bone,  221 


Hyoid,  helix,  31 1 
Hypermastia,  236 
Hyperthelia,  236 
Hypertrichosis,  234 
Hypogastric  artery,  193 
Hypoglossal  nerve,  282 
Hypophysis,  262 
Hypospadias,  168 
Hypothalamus,  260 
Hypotrichosis,  234 

Ichthyoid  blood  cells,  171 
Ichthyosis,  231 

Ileum,  diverticulum  of,  52,  109 
Iliac  artery,  193,  194 
vein,  200 
Ilium,  222 

Imperforate  anus,  109 
Implantation  of  ovum,  56 
Incisive  canal  (of  Stenson),  89 
foramen,  89 

Increase  in  surface  area,  82 
Incus,  221,  310 
Infancy,  81 
Inferior  concha,  297 
Infracardiac  bronchus,  1 1 7 
bursa,  121 
Infundibulum,  261 
Inguinal  canal,  160 
crest,  160 
fold,  158,  160 
hernia,  163 
Inheritance,  28 
cytoplasmic,  29 
Inner  cell  mass,  33 
enamel  cells,  41 
epithelial  mass  of  gonad,  150 
Innominate  artery,  189 
vein,  197 
Insemination,  24 
Instruments,  dissecting,  405 
Insula,  272 

Integumentary  system,  230 
Interatrial  foramen,  176 
Intercalated  discs,  224 
Intercostal  artery,  19 1 
vein,  200 

Intermediate  cell  mass,  8 
cords,  153,  154 
Intermenstruum,  60 
Internal  capsule,  265 
ear,  305 

sexual  transformations,  153 
Interosseous  artery,  194 
Intersegmental  artery,  186 
fiber  tracts,  237 
Interstitial  cells  of  testis,  154 
Interventricular  foramen,  of  heart,  183 
closure  of,  183 
of  Monro,  266 
septum,  183 
sulcus,  183 

Intervertebral  discs,  214 
muscles,  226 

Intervillous  circulation,  66 
spaces,  64 

Intestinal  glands,  107 
loop,  106,  124 


420 

Intestinal  portal,  of  chick,  318 
Intestine,  106 
anomalies  of,  109 
glands  of,  107 
pig- 

villi  of,  107 

Intracartilaginous  ossification,  210 
lntrameml)ranous  ossification,  208 
Intra-ocular  vessels,  302 
Introduction,  i 
Invagination,  8 
Iris,  304 

muscles  of,  304 
Ischium,  222 
Island  of  Reil,  272 
Islands,  blood,  169 
of  pancreas,  1 13 
Isolecithal  ova,  13 

Jacohson's  organ,  279,  296 
Jaw,  lower,  77 
upper,  78 
Joint  cavity,  21 1 
Joints,  2 1 1 

Jugular  ganglion,  286 
lymph  sacs,  203 
veins,  199 

Keratin  fibrils,  232 
Kcratohyalin,  231 
Kidney,  139 

anomalies  of,  144 
cystic,  145 
horse-shoe,  144 
tubules  of,  139,  142 
Knot,  primitive  (of  Hensen),  39 

Labia  majora,  166 
minora,  166 
Labial  groove,  89 
ligament,  100 

Labio-scrotal  swellings,  163 
Laboratory  manual,  313 
Labyrinth,  bony,  309 
membranous,  309 
Lacrimal  bone,  220 
duct,  305 
glands,  304 
Lacunae,  blood,  37 
bone,  209 

Lamellatcd  corpuscles,  292 
Lamina,  dental,  89 
perpendicularis,  218 
terminalis,  266 
Langhans’  layer,  57,  62 
Lanugo  hair,  234 
Large  intestine,  107 

mononuclear  leucocytes,  1 72 
Laryngeal  nerves,  recurrent,  190 
Laryngo-tracheal  groove,  114,  115 
Larynx,  114,  115 
cartilages  of,  116 
muscles  of,  1 16,  229 
ventricles  of,  1 1 6 
Lateral  fissure,  272 
lemniscus,  260 


INDEX 

Lateral  line  organs,  287 
nasal  process,  77,  294 
palatine  process,  87 
pharyngeal  recess,  100 
recesses,  256 
swellings  of  tongue,  96 
umbilical  ligament,  193 
ventricles,  252,  265,  270 
Law,  Mendel's,  of  heredity,  28 
of  biogenesis,  9 
of  genetic  restriction,  9 
of  recapitulation,  9 
Layer,  chorioid,  of  eye,  304 
compact,  of  uterus,  61,  62 
enamel,  91 
ependymal,  246 
epitrichial,  231 

ganglion  cell,  of  retina,  279,  301 

germ,  5 

granular,  259 

Langhans’,  57,  62 

mantle,  248 

marginal,  249 

medullary,  259 

molecular,  259 

nerve  fiber,  of  retina,  279,  301 
nervous,  of  retina,  299 
pigment,  of  retina,  299 
sclerotic,  304 

spongy,  of  uterus,  61,  62,  64 
Lecithin,  12 
Leg,  80 

Lemniscus,  lateral,  260 
median,  257,  260 
Lens  of  eye,  301 
capsule,  302 
epithelium,  301 
fibers,  301 
placode,  297 
stars,  302 

suspensory  ligament  of,  302 
vascular  tunic  of,  303 
vesicle,  297,  301 
Lenticular  nucleus,  265 
Lesser  curvature,  of  stomach,  104 
omentum,  104,  126 
peritoneal  sac,  121 
Leucocytes,  171 
granular,  172 
mast,  173 

mononuclear,  large,  172 
non-granular,  172 
origin  of,  171 
polymorphonuclear,  172 
Ligament,  207 
broad,  160 
coronary,  1 26 
diaphragmatic,  160 
falciform,  126 
gastro-colic,  124 
gastro-splenic,  124 
I hepato-duodenal,  126 
hepato-gastric,  126 
of  internal  genitalia,  139 
of  liver,  126 
of  testis,  1 61 
of  vertebral  column,  213 
proper,  of  ovary,  139 


INDEX 


421 


Ligament,  round,  of  liver,  126,  196,  203 
of  uterus,  160 
spheno-mandibular,  221 
spleno-renal,  124 
stylo-hyoid,  221 
suspensory,  of  lens,  302 
triangular,  126 
umbilical,  lateral,  193,  203 
middle,  147,  203 
Ligamentum  arteriosum,  203 
labiale,  160 
ovarii,  159 
scroti,  i6i 

teres,  of  liver,  126,  196,  203 
of  uterus,  160 
testis,  16 1 
venosum,  197,  203 
Limb  buds,  80 
muscles  of,  227 
Limbs,  80 
Limbus  ovalis,  179 
Limiting  membranes,  of  retina,  301 
Line,  epiphyseal,  21 1 
lateral,  287 
milk,  235 

Lingual  tonsil,  100 
Lip,  78,  87,  294 
hare,  79 
rhombic,  257 
Liquor  folliculi,  20 
Liver,  109 

anomalies  of,  u 2 
capsule  of,  126 
caudate  lobe  of,  126 
cords,  no 

coronary  appendages  of,  126,  132 
ducts,  1 1 1 
ligaments  of,  126 
lobules  of,  1 12 
quadrate  lobe  of,  126 
sinusoids  of,  no 
Lobes,  of  cerebrum,  270 
olfactory,  266 
Lobule,  of  ear,  312 
Lobules,  of  liver,  112 
Lobuli  epididymidis,  156 
Loop,  intestinal,  106 
Henle’s,  142 
Stoerck’s,  143 
Lower  jaw,  77 
Lumbar  arteries,  19 1 
enlargement,  250 
veins,  200 

Lumbo-sacral  plexus,  278 
Lung,  114,  116 
alveoli  of,  118 
anomalies  of,  118 
apical  buds,  116 
buds,  1 14 

changes  at  birth,  118 
eparterial  bud,  1 16 
pig-  358,  381 

stem  buds,  116 
Lunula,  232 
Lymph  glands,  204 
sacs,  203 
Lymphatics,  203 
peripheral,  203 


Lymphocytes,  170 
primary,  1 70 
Lymphoid  tissue,  204 

Macacus,  cleavage  in,  34 
Maculffi  acusticae,  307 
Magma  reticulare,  70 
Mali’s  pulmonary  ridge,  129,  190 
Malleus,  221,  310 
muscle  of,  310 
Mammals,  10 
cleavage  in,  33 
gastrulation  in,  36 
origin  of  mesoderm  in,  40 
of  notochord  in,  41 
Mammary  artery,  19 1 
gland,  235 

anomalies  of,  236 
rudimentary,  236 
supernumerary,  236 
Mammillary  bodies,  261 
recess,  261 
Mandible,  220 
Mandibular  arch,  96 
foramen,  221 
nerve,  284 
process,  77,  220,  294 
Mantle  layer,  248 
zone,  238 
Manubrium,  216 
Marginal  layer,  249 
fibers  of,  249 
sinus,  65 
zone,  238 

Alarrow,  bone,  210 
red,  209 
yellow,  210 
cavity,  210 
tissue,  210 
Marsupials,  10 
Massa  intermedia,  262 
Mast  leucocytes,  173 
Mastication,  muscles  of,  228 
Mastoid  cells,  219 
process,  219 
Maternal  placenta,  63 
Maturation,  16 
equational,  19 
of  Ascaris  sperm,  17 
of  chick  ovum,  313 
of  human  ovum,  22 
sperm,  19 
of  mouse  ovum,  22 
reductional,  18 
significance  of,  28 
Maxilla,  220 
Maxillary,  nerve,  284 
process,  77,  220,  294 
sinus,  297 

Maxillo-turbinal,  297 
Meatus,  external  acoustic,  310 
Aleckel’s  cartilage,  220 
diverticulum,  52,  109 
Meconium,  109 
Median  artery,  194 
lemniscus,  257,  260 
longitudinal  bundle,  257 
nasal  process,  78,  294 


422 

IMedian  nerve,  278 
palatine  process,  89 
Mediastinum,  117 
of  ovary,  1 54 
of  testis,  I 54 
Medulla  oblongata,  252 
of  hair,  234 
of  kidney,  141 
Aledullary  cavity,  210 
layer,  of  cerebellum,  259 
sheath,  244 
velum,  259 
Alegakaryocyte,  173 
Alegaloblast,  171 
Meibomian  glands,  304 
Melanism,  231 
Alembrane,  anal,  85,  145 
basilar,  309 
bone,  208 

(levelo])ment  of,  208 
bones  of  skull,  219 
cloacal,  85,  145 
decidual,  38 

separation  of,  at  birth,  66 
fetal,  44 
birds,  44 
mammals,  46 
man,  49 
rejjtiles,  44 

limiting,  of  retina,  301 
nuclear,  12 
obturator,  222 
peridental,  93 
pharyngeal,  85 
pleuro-pericardial,  128,  129 
pleuro-peritoneal,  128,  129 
pupillary,  303,  304 
Reissner’s,  309 
synovial,  21 1 
tectorial,  307 
tympanic,  31 1 
urethral,  163 
urogenital,  83,  146 
vestibular,  309 
vitelline,  14 

A'lembranous  labyrinth,  309 
stage  of  skeleton,  212 
urethra,  147 

Alendel’s  law  of  heredity,  28 
Meningoc(X“le,  273 
A'leningoencephalocoele,  273 
A'lenstruation,  39 

relation  to  ovulation,  2J\ 
A'leroblastic  ovum,  30 
Mesameboids,  170 
A'lesencephalon,  231,  259 
Alesenchyme,  5,  206 
chick,  320 

Mesenteric  artery,  inferior,  189,  193 
superior,  187,  189,  193 
vein,  superior,  194 
Mesentery,  119 
anomalies  of,  127 
caval,  121,  200 
vein  of,  200 
dorsal,  120 

differentiation  of,  120 
pig-  360,  383 


INDEX 

Mesentery,  primitive,  119 
proper,  12 1,  125 
torsion  of,  125 
ventral,  123 

differentiation  of,  123 
A'lesocardium,  126 
dorsal,  126,  173 
ventral,  123 
Mesocolon,  121,  125 
Alesoderm,  5,  34 
amphibia,  37 
Amphioxus,  37 
birds,  39 

chick,  314,  320,  330,  340 
extra-embryonic,  42 
intra-embryonic,  42 
mammal,  40 
origin  of,  36 
])rimary,  37 
reptiles,  38 
somatic,  8,  41 
splanchnic,  8,  41 
Tarsiixs,  42 

Mesodermal  derivatives,  6,  119 
segments,  7 
cavities  of,  128 
chick,  316,  330 
Mesoduodenum,  12 1 
A'lesogastrium,  dorsal,  121 
A'lesonephric  duct,  139,  130 
transformation  of,  156 
fold,  137 
tubules,  137 

transformation  of,  156 
Alesonephros,  133,  137 
chick,  331,  340,  332 
diaphragmatic  ligament  of,  i6o- 
pig,  360,  383 
Alesorchium,  130,  160 
Mesorectum,  121 
Mesosalpinx,  139 
Mesothelium,  8 
A'lesovarium,  130,  159 
A'letacarpus,  222 
Metameres,  7 
Aletamerism,  7 
A'letanephros,  133,  139 
calyces  of,  141 
chick,  332 

collecting  tubules,  139,  141 
cortex,  1 41 
pelvis,  139 
pig,  360,  383 
tubules,  139,  142 
’ ureter,  139.  141 
Metaphase  of  mitosis,  4 
A'letatarsus,  222 
Metathalamus,  260 
! Metencephalon,  251,  258 
A'licropyle,  26 
Mid-brain,  251 
Middle  concha,  297 
ear,  310 
Mid-gut,  85 
chick,  337 
I Milk  ducts,  236 
line,  235 
teeth,  90 


INDEX 


423 


Milk  teeth,  periods  of  eruption  of,  93 
witch,  236 
Mitochondria,  12 
Mitosis,  3 

significance  of,  28 
Mitotic  figure,  4 
Mitral  cells,  279 
valve,  18 1 
Modiolus,  301 

Molecular  layer,  of  cerebellum,  259 

Moles,  231 

Moll’s  glands,  304 

Monad,  17 

Monaster,  45 

Mononuclear  leucocytes,  large,  172 
Monophyletic  theory  of  blood,  169 
Monotremes,  10 
Monro’s  foramen,  266 
Mons  pubis,  166 
Monsters,  double,  43 
Montgomery’s,  glands,  226 
Morphogenesis  of  central  nervous  system,  245 
of  muscle,  224 
of  skeleton,  212 
Morula,  30 

Motor  nerves,  somatic,  282 
nuclei,  257,  260 

Mouse  ovum,  fertilization  of,  26 
maturation  of,  22 
Mouth  cavity,  85 
Mullerian  ducts,  150 

transformation  of,'! 57 
Muller’s  fibers,  301 
tubercle,  147,  157 
Multiplication,  cell,  3 
Mummified  fetuses,  76 
Muscles,  223 

anomalies  of,  229 
arrector  pili,  234 
cardiac,  223 
ciliary,  304 
columns,  224 

fundamental  processes,  225 

histogenesis  of,  223 

intervertebral,  226 

of  abdomen,  226 

of  auditory  ossicles,  310 

of  back,  226 

of  expression,  229 

of  extremities,  227 

of  eyeball,  228,  229 

of  hair,  234 

of  head,  228 

of  iris,  223,  304 

of  larynx,  229 

of  limbs,  227 

of  malleus,  310 

of  mastication,  228 

of  neck,  226 

of  palate,  229 

of  perineum,  227 

of  pharynx,  229 

of  stapes,  310 

of  sweat  glands,  223,  235 

of  thorax,  226 

of  tongue,  229 

of  trunk,  226 

papillary,  183 


Muscles,  plate,  225 
pupillary,  304 
skeletal,  223,  224 
smooth,  223 
stapedial,  310 
tensor  tympani,  310 
j thoraco-abdominal,  226 
Muscular  system,  223 
Musculature,  skeletal,  225 
visceral,  224 

Musculocutaneous  nerve,  278 
j Myelencephalon,  252,  254 
i Myelin,  244,  250 
I sheath,  244 
I Myelinated  nerve  fibers,  245 
j Myeloccele,  273 
I Myelocytes,  172 
Myoblasts,  223 
j Myocardium,  173 
Myofibrils,  223,  224 
Myoglia,  223 
I Myotome,  224 

changes  in  during  morphogenesis,  226 

N.evi,  231 
Nail  fold,  231 
plate,  232 
Nails,  231 

Nares,  external,  294 
Nasal  bone,  220 
passages,  87 
processes,  78,  294 
septum,  218,  295 
Naso-lacrimal  duct,  305 
groove,  305 
Naso-turbinal,  297 
Navel,  54 
Neck,  76 
muscles  of,  226 
of  spermatozoon,  15 
Neonatal  period,  81 
Neopallium,  267 
Nephrogenic  cords,  137,  140 
tissue,  differentiation  of,  142 
Nephrostome,  135 
Nephrotome,  8 
Nerve,  274 
abducens,  282 
accessorius,  285 
acoustic,  280 
afferent  fibers  of,  241 
ansa  hypoglossi,  278 
axillary,  278 
cells,  237 

chorda  tympani,  285 
cranial,  278 
efferent  fibers  of,  239 
facial,  284 
femoral,  278 
fibers,  274 

cell  chain  theory,  245 
classification  of,  275 
layer,  of  retina,  279,  301 
myelinated,  243 
unmyelinated,  245 
glossopharyngeal,  285 
hypoglossal,  282 
mandibular,  284 


424 


INDEX 


Nerve,  maxillary,  284 
median,  278 
motor  somatic,  282 
musculocutaneous,  278 
obturator,  278 
oculomotor,  282 
olfactory,  271) 
ophthalmic,  284 
optic,  279,  ,^01 
jjeroncal,  278 
petrosal,  suijcriicial,  285 
jdircnic,  278 
Ijlexus,  278 
radial,  278 

recurrent  laryngeal,  190 
sciatic,  278 
sensory,  special,  279 
somatic,  275 
motor,  282 
S])i!ial,  275 

accessory,  285 
terminal,  279 
terminations,  free,  292 
tibial,  278 
trigeminal,  283 
troehlear,  282 
ulnar,  278 
vagus,  285 
visceral,  275 
mixed,  283 

Nervous  layer,  of  retina,  299 
system,  central,  237 

chick,  327,  337,  350 
pig,  355,  ,37« 

])cripheral,  274 
sympathetic,  287 
tissue,  histogenesis  of,  237 
Neural  crest,  241 
folds,  237 
groove,  237 
chick,  316 
plate,  237 
tul)e,  36,  237 

anomalies  of,  273 
chick,  319 

derivatives  of,  254 
origin  of,  36,  237 
su]jporting  elements  of,  242 
Ncurenteric  canal,  38,  41 
Neurilemma,  244 
Neurobiotaxis,  287 
Neuroblasts,  239 

differentiation  of,  239 
of  retina,  301 
Neurofibrillae,  239 
Neuroglia  cells,  237,  239,  242 
fibers,  239,  242 
Neurokeratin,  244 
Neuromeres,  229,  257 
Neuron,  239 
afferent,  241 
doctrine,  245 
efferent,  239 
Neuropore,  246 
Neutrophils,  172 
'Newborn,  81 
Nipple,  236 

supernumerary,  236 


Node,  primitive,  39 
of  Ranvier,  244 
Nodose  ganglion,  286 
Nodulus  cerebelli,  259 
Non-granular  leucocytes,  172 
Normoblasts,  171 
Nose,  78,  293 
apex  of,  78 
bridge  of,  78 
septum  of,  78 

Notochord,  10,  42,  212,  216 
origin  of,  36,  38,  41 
Notochordal  canal,  40 
plate,  37,  38,  39,  41 
Nuck’s  diverticula,  163 
Nuclear  membrane,  12 
Nuclei  ])ulposi,  42,  212 
Nucleolus,  12 
Nucleus,  12 
ambiguus,  285 
caudate,  265 
cuneatus,  257 
dentate,  259 

dorsal  motor,  of  vagus,  285 

gracilis,  257 

lenticular,  265 

motor,  257,  260 

of  pons,  258 

olivary,  257 

receptive,  257 

red,  260 

terminal,  257 

Obex,  257 

Oblique  vein  of  left  atrium,  197 
Obturator  membrane,  222 
nerve,  278 
Occipital  bone,  218 
lobe,  270 

Oculomotor  nerve,  282 
Odontoblasts,  92 
Oestrus,  22 

Olfactory  apparatus,  264,  266 
brain,  264 
bullj,  266 
epithelium,  295 
fossa,  294 
lobe,  266 
nerve,  279 
organ,  293 
pits,  77,  294 
placodes,  294 
tract,  266 

Olivary  nucleus,  257 
Olive,  258 
Omental  bursa,  12 1 
inferior  recess  of,  12 1 
Omentum,  greater,  104,  121 
lesser,  104,  126 
Oocyte,  20 
primary,  20 
secondary,  21 
Oogenesis,  16,  19 
Oogonia,  20 
Ootid,  21 
Operculum,  272 
Ophthalmic  artery,  191 
nerve,  284 


INDEX 


42 


Optic  chiasma,  266,  279 
cup,  297,  298 
nerve,  301 
placode,  298 
stalk,  297 
tract,  279 
vesicle,  251,  297 
Ora  serrata,  299 
Oral  cavity,  85 
fossa,  85 

Organ,  Corti’s,  307 
enamel,  89 
Jacobson’s,  279,  296 
lateral  line,  287 
sense,  79 
spiral,  307 

vomero-nasal,  279,  296 
Organogenesis,  85 
Ossicles,  auditory,  310 
Ossification  centers,  2 1 1 
endochondral,  210 
intracartilaginous,  210 
intramembranous,  208 
of  carpus,  221 
of  chondrocranium,  218 
of  clavicle,  221 
of  coxal  bone,  222 
of  ethmoid  bone,  218 
of  femur,  222 
of  fibula,  222 
of  frontal  bone,  220 
of  hip  bone,  222 
of  humerus,  221 
of  hyoid  bone,  221 
of  ilium,  222 
of  incus,  221 
of  ischium,  222 
of  lacrimal  bone,  220 
of  malleus,  221 
of  mandible,  220 
of  maxillary  bone,  220 
of  metacarpus,  222 
of  metatarsus,  222 
of  nasal  bone,  220 
of  occipital  bone,  218 
of  palate  bone,  220 
of  parietal  bone,  220 
of  patella,  222 
of  phalanges,  222 
of  pisiform,  222 
of  pubis,  222 
of  radius,  221 
of  ribs,  215 
of  scapula,  221 
of  skull,  217 
of  sphenoid  bone,  218 
of  stapes,  221 
of  sternum,  216 
of  tarsus,  222 
of  temporal  bone,  219 
of  tibia,  222 
of  ulna,  221 
of  vertebrae,  215 
of  vomer,  220 
of  zygomatic  bone,  220 
perichondral,  209 
periosteal,  209,  210 
Osteoblast,  93,  208 


Osteoclast,  209 
Otic  ganglion,  280 
vesicle,  305 
Otoconia,  307 
Otocyst,  305 

chick,  329,  337,  350 
_ pig-  350,  379 
Outer  enamel  cells,  90 
Ovarian  artery,  192 
pregnancy,  67 
vein,  200 
Ovary,  154 
anomalies,  156 
descent  of,  161 
differentiation  of,  154 
follicles  of,  23,  156 
mediastinum,  134 
Pfitiger’s  tubes  of,  156 
proper  ligament  of,  159 
septula  of,  154 
stroma  of,  136 
tunica  albuginea  of,  153 
Ovulation,  22 

relation  to  menstruation,  24 
Ovum,  12,  14 

centrolecithal,  14 
chick,  313 

cleavage  of,  30.  See  also  Cleavage. 

comparison  with  spermatozoon,  15 

fertilization  of,  25 

holoblastic,  30 

implantation  of,  56 

isolecithal,  13 

maturation  of,  22 

meroblastic,  30 

period  of,  81 

segmentation  of,  30.  See  also  Cleavage. 
structure  of,  12 
telolecithal,  14 

Pains,  66 
after,  66 
Palate,  87,  294 
anomalies  of,  89 
bones,  220 
cleft,  89 
hard,  89 
muscles  of,  229 
premaxillary,  294 
primitive,  294 
soft,  89 

Palatine  processes,  lateral,  87 
median,  89 
tonsil,  100 
Pallium,  264,  266 
Pancreas,  112 

accessory  duct  of,  1 1 3 
alveoli  of,  113 
anomalies  of,  113 
dorsal,  112 
duct  of,  1 1 3 
islands  of,  113 
ventral,  1 12 
Pancreatic  duct,  113 
Papillae,  dental,  89,  92 
hair,  233 
renal,  14 1 
tongue,  96,  97,  293 


INDEX 


42b 


Papillary  duel,  14 1 
muscles,  183 
Paradidymis,  156 
Paraganj^lia,  289 
Paranasal  sinuses,  297 
Parathyroid  gland,  loi 
Para-urethral  ducts,  148 
Parietal  hone,  220 
lobe,  270 
pleura,  118 

Parieto-oeeipital  fissure,  272 
Parolfactory  area,  267 
Paroo]ihoron,  157 
Parotid  gland,  <)7 
Pars  e;cca,  2<)<) 
ciliaris,  299 
intermedia,  263 
iridica,  291) 

lateralis,  of  sacrum,  2ih 
ojitica,  2i)(^ 
radiata,  141 
tuberalis,  2(")3 
Parthenogeni'sis,  23 
artificial,  26 
Parturition,  66 
Patella,  222 

Peduncles  of  cerebrum,  260 
Pelvis,  renal,  139 
Penis,  I (>4 

Perforated  substance,  anterior,  2(>7 
Pericardial  cavity,  127,  133 
Pericardium,  132 
Perichondral  ossification,  210 
Pericliondrium,  208 
Peridental  membrane,  93 
Periderm,  230 
Perilymph  spaces,  309 
Perineum,  146,  163 
muscles  of,  227 
Period,  developmental,  i,  81 
neonatal,  81 
of  emlrryo,  69,  81 
of  fetus,  73,  81 
erf  gestation,  69 
of  newborn,  81 
of  ovum,  81 

Periosteal  ossification,  209,  210 
Periosteum,  209,  210 
Periotic  capsule,  217 
Peri4iheral  lymphatics,  203 
nervous  system,  274 
pis.  355 
sinus,  204 

Peritoneal  cavity,  128,  133 
sac,  lesser,  1 2 1 
Permanent  teeth,  93 

periods  of  eruption  of,  94 
Peroneal  artery,  194 
nerve,  278 

Petrosal  ganglion,  283 
nerve,  su]>crficial,  283 
Pfiuger's  tubes,  136 
Phalanges,  222 
Phallic  urethra,  146 
Phallus,  163 
Pharyngeal  bursa,  100 
membrane,  83 
pouches,  99 


Pharyngeal  pouches,  chick,  329,  338,  357 
pig.  ,357.  380 
tonsil,  100 

Pharyngo-palatine  arches,  89 
Pharynx,  99 
muscles  of,  229 
pig.  357.  379 
Philtrum,  78 
Phrenic  artery,  192 
nerve,  278 
Pig  embryos,  333 

dissection  methods,  403 
early  stages,  353 
eighteen  mm.  stage,  400 
six  mm.  stage,  354 

transverse  sections,  367 
ten  to  twelve  mm.  stage,  376 
transverse  sections,  387 
thirty-five  mm.  stage,  402 
Pigment  granules,  231 
layer  of  retina,  299 
i Pillars  of  Corti,  307 
Pineal  body,  260 
Pisiform  bone,  222 
Pit,  olfactory,  77,  294 
Pituitary  body,  262 
Placenta,  44,  48,  62,  67 
accessory,  67 
cotyledons  of,  65 
fetalis,  62 

intervillous  spaces  of,  64 
circulation  of,  66 
mammalian,  47 
materna,  63 
position  of,  67 
prsevia,  67 
septa,  64 
succenturiate,  67 
vessels  of,  63 
yolk  sac,  46,  48 
Placentalia,  10 
Placentation,  48 
Placodes,  287 
auditory,  303 
lens,  297 
olfactory,  294 
optic,  298 
Plan,  body,  10 

Plasm,  germ,  continuity  of,  9 
Plate,  alar,  248 
basal,  63,  248 
closing,  99 
cribriform,  219 
cutis,  231 
floor,  246 
nail,  232 
neural,  237 

notochordal,  37,  38,  39,  41 
roof,  246 

Platelets,  blood,  173 
Pleura,  ])arietal,  118 
visceral,  118 

Pleural  cavity,  128,  129,  133 
Pleuro-pericardial  canal,  128 
cavity,  128 
membranes,  128,  129 
Pleuro-pcritoneal  canal,  128 


INDEX 


427 


Pleuro-peritoneal  canal,  cavities,  127 
membranes,  128,  129 
Plexus,  brachial,  278 
cervical,  278 
chorioid,  257,  260,  267 
lumbo-sacral,  278 
nerve,  278 
prevertebral,  288 
Plica  semilunaris,  304 
Polar  bodies,  21 
Pole,  animal,  14 
vegetal,  14 
Polocytes,  21 
Polydactyly,  81,  222 
Polymorphonuclear  leucocytes,  1 72 
Polyphyletic  theory  of  blood,  169 
Polyspermy,  26 
Pons,  252 

nuclei  of,  258 
Pontine  flexure,  254 
Popliteal  artery,  194 
Portal,  intestinal,  chick,  318 
vein,  194 
Postanal  gut,  86 
Postbranchial  bodies,  102 
Postcardinal  vein,  187,  199 
Posterior  commissure,  166 
lymph  sacs,  203 
Postnatal  development,  81 
Pouches,  branchial,  99 
coelomic,  36 
pharyngeal,  99 
Rathke’s,  262 
Seesel’s,  100 

Precardinal  vein,  187,  197 
Precartilage,  208 
Preformation,  doctrine  of,  2 
Pregnancy,  abdominal,  67 
duration  of,  69 
ectopic,  67 
extra-uterine,  67 
glands  of,  61 
ovarian,  67 
plural,  67 
tubal,  67 
twin,  67 

Premaxillary  palate,  294 
Prenatal  development,  69,  81 
Prepucium,  165 
Prevertebral  plexuses,  288 
Primary  chorionic  villi,  54,  57 
excretory  ducts,  135,  137 
lymphocyte,  170 
mesoderm,  37 
oocyte,  20 
spermatocyte,  16 
Primates,  10 
cleavage  in,  34 
gastrulation  in,  36 
origin  of  mesoderm  in,  42 
Primitive  blood  cells,  169,  170 
choanae,  87,  294 
coelom,  127 
genital  ducts,  150 
groove,  39,  40,  70 
knot  or  node,  39 
mesentery,  119 
palate,  294 


Primitive  pit,  40 
segments,  7 
streak,  38,  40,  70 
of  chick,  314 
vascular  system,  185 
Primordial  follicles,  20,  156 
germ  cells,  16,  149 
Proamnion  of  chick,  319 
Process,  articular,  215 
coracoid,  221 
costal,  214 
fronto-nasal,  77,  294 
head,  39,  40,  42 
chick,  315 

lateral  nasal,  77,  294 
palatine,  87 

mandibular,  77,  200,  294 
mastoid,  219 
maxillary,  77,  220,  294 
median  nasal,  78,  294 
palatine,  89 
nasal,  78,  294 
styloid,  219,  221 
transverse,  215 
vermiform,  108 
xiphoid,  216 
cleft,  222 

Processes,  developmental,  8 

fundamental,  in  myogenesis,  225 
Processus  globulares,  88 
vaginalis,  162 
Proctodeum,  85 
Pronephric  duct,  135,  137 
tubules,  135 
Pronephros,  135 
chick,  331 

Pronucleus,  female,  22 
male,  26 

Proper  ligament  of  ovary,  159 
Prophase  of  mitosis,  3 
Prosencephalon,  231 
Prostate  gland,  148 
Prostatic  urethra,  147 
utricle,  157 
Pseudocopulation,  24 
Puberty,  81 
Pubis,  222 

Pulmonary  arteries,  118,  181 
ridge,  129,  190 
vein,  1 18,  18 1 
Pulp,  dental,  92 
enamel,  90 
Pulpy  nuclei,  42,  212 
Pupillary  membrane,  303,  304 
anomalies  of,  305 
muscles,  304 
Purkinje  cells,  259 
Pyloric  sphincter,  105 
Pyramidal  cells,  273 
Pyramids,  of  brain,  238 
of  kidney,  141 

Quadrate  lobe  of  liver,  126 

Rachischisis,  273 
Radial  artery,  194 
nerve,  278 
Radius,  221 


428 

Ramus  angularis,  196 
arcuatus,  196 
communicans,  277,  287 
dorsal,  277 
gray,  287 
terminal,  277 
white,  287 

Ranvier’s  nodes,  244 
Raphe,  of  scrotum,  164 
Rathke’s  pouch,  262 
Recapitulation,  law  of,  9 
Receptive  nuclei,  257 
Recess,  inferior,  of  omental  bursa,  12 1 
lateral,  256 
mammillary,  261 
Recessiveness,  29 
Rectum,  85,  108,  143 
Red  blood  corpuscles,  1 7 1 
bone  marrow,  209 
nucleus,  260 
Reductional  mitosis,  18 
Reference,  titles  for,  1 1 
Reichert’s  cartilage,  310 
Reil’s  island,  272 
Reissner’s  membrane,  309 
Renal  artery,  192 
columns,  141 
corpuscles,  139,  142 
papillae,  141 
pelvis,  139 
pyramid,  14 1 
tubules,  142 
vein,  200 
Reptiles,  10 
cleavage  in,  32 
gastrulation  in,  35 
origin  of  mesoderm  in,  37 
of  notochord  in,  38 
Residual  lumen,  263 
Respiratory  epithelium,  117,  296 
system,  1 14 
chick,  351 
pig,  357,  379 
Rete  ovarii,  154,  157 
testis,  153,  156 
Reticular  formation,  257 
tissue,  206 
Retina,  299 

nervous  layer  of,  299 
pigment  layer  of,  299 
Retroperitoneal  lymph  sac,  203 
Rhinal  fissure,  271 
Rhinencephalon,  264,  266 
Rhombencephalon,  251 
Rhombic  grooves,  257 

lip,  237 
Ribs,  213 

anomalies  of,  222 
Ridge,  arytenoid,  96 
dental,  89 
pulmonary,  129 
Rod  cells,  301 
Rolando’s  fissure,  272 
Roof  plate,  246 
Roots,  spinal,  273 
Rotation  of  stomach,  104 
Round  ligament,  of  uterus,  1 60 
Rupture  of  follicle,  23 


INDEX 

Sac,  dental,  93 
lymph,  203 
Sacculus,  306 
Saccus  vaginalis,  162 
Sacral  artery,  middle,  193 
Sacrum,  216 

pars  lateralis  of,  216 
Salivary  glands,  97 
Santorini,  duct  of,  113 
! Saphenous  vein,  200 
■ Sarcoplasm,  224 
Satyr  tubercle,  3 1 1 
Sauroid  blood  cells,  171 
Scala  tympani,  309 
vesti'buli,  309 
Scapula,  221 
Schwalbe’s  tubercle,  31 1 
Sciatic  artery,  194 
nerve,  278 
Sclera,  304 
Sclerotome,  214 
Scrotum,  164 
ligament  of,  161 
raphe  of,  164 
septum  of,  164 
Sebaceous  glands,  234 
Secondary  choanas,  87 
chorionic  villi,  34,  37 
oocyte,  21 
spermatocyte,  16 
Secretory  tubule,  139 
Sections,  chick,  of  five  segments,  319 
of  head  process  stage,  316,  317 
of  primitive  streak  stage,  313 
of  seven  segments,  322 
of  seventeen  segments,  331 
of  three  to  four  days,  331 
of  twenty-seven  segments,  342 
pig,  6 mm.,  367 
10  mm.,  387 
Seessel’s  pouch,  100 
Segmentation  of  body,  7 

of  ovum,  30.  See  also  Cleavage. 
of  vertebrate  head,  229 
Segments,  mesodermal,  7 
chick,  316,  321 
primitive,  7 
Semen,  24 

Semicircular  ducts,  306 
Semilunar  ganglion,  283 
valves,  181 

Seminal  colliculus,  139 
fluid,  24 
vesicle,  137 
Sense  cells,  292 

organs,  79,  292,  330 
chick,  327,  337 
pig.  355.  37« 

Sensory  epithelium,  237 

nerves,  special  somatic,  279 
organs,  general,  292 
Septa  placentae,  64 
Septula  ovarii,  134 
Septula  testis,  134 
Septum,  atrial,  176 
interventricular,  183 
median,  248 
membranaceum,  183 


INDEX 


Septum,  nasal,  78,  218,  295 
pellucidum,  279 
space  of,  270 
primum,  176 
scroti,  164 
secundum,  176 
spurium,  177 
transversum,  109,  128 
Sertoli,  sustentacular  cells  of,  16, 
Sesamoid  bones,  212 
Sex  chromosome,  29 
determination,  29 
Shaft  of  hair,  234 
Sheath  cells,  244 
epithelial,  91 
hair,  234 
medullary,  244 
myelin,  244 
Shoulder  blade,  221 
Sigmoid  colon,  108 
mesocolon,  125 
Sinus,  cervical,  77 
coronary,  179,  197 
frontal,  297 
marginal,  65 
maxillary,  297 
paranasal,  297 
peripheral,  204 
sphenoidal,  297 
terminal,  chick,  327 
urogenital,  85,  145 
venosus,  174,  177 
valves  of,  177,  179 
Sinusoids  of  liver,  no,  194 
Situs  viscerum  inversus,  118 
Skeletal  muscle,  223,  224 
musculature,  225 
system,  206 
Skeleton,  212 

anomalies  of,  222 
appendicular,  221 
axial,  212 

blastemal  stage,  212 
branchial  arch,  220 
membranous  stage,  212 
Skene’s  ducts,  148 
Skin,  230 

anomalies  of,  231 
corium  of,  231 
derma  of,  231 
epidermis  of,  230 
Skull,  216 

chondrification  of,  217 
membrane  bones  of,  219 
ossification  of,  217 
Small  intestine,  106 
Smooth  muscle,  223 
Soft  palate,  89 
spots,  220 

Solitary  tract,  257,  285,  287 
Soma,  9 

Somatic  mesoderm,  8 
nerves,  275 

Somatopleure,  8,  37,  41 
Somites,  7 
Sperm  cell,  14 
Spermatic  artery,  192 
cord,  163 


429 


17,  154 


Spermatic  vein,  200 
Spermatid,  16 
Spermatocyte,  primary,  16 
secondary,  16 
Spermatogenesis,  16 
Spermatogonia,  16,  154 
Spermatozoon,  12,  14 
atypical,  13 

comparison  with  ovum,  15 
life  of,  25 
locomotion  of,  25 
maturation  of,  19 
structure  of,  15 
Sphenoid  bone,  218 
Sphenoidal  sinus,  297 
Spheno-mandibular  ligament,  221 
Sphincter,  pyloric,  105 
Spina  bifida,  273 
Spinal  accessory  nerve,  285 
artery,  190 
cord,  246 

anomalies  of,  273 
I ganglia,  241 
I supporting  cells  of,  241 
nerves,  275 
rami  of,  277 
Spinal  roots,  275 

tract,  descending,  of  trigeminal  nerve,  257, 
283 

I Spiral  ganglion,  280 
limbus,  307 
! organ,  307 

; sulcus,  307 

tunnel,  307 
Spireme,  4 

Splanchnic  mesoderm,  8 
Splanchnopleure,  8,  37,  41 
Spleen,  205 

Splenic  corpuscles,  205 
Spleno-renal  ligament,  124 
Spongioblasts,  239 
Spongy  layer  of  uterus,  61,  62,  64 
1 Stalk,  optic,  297 
I Stapedial  artery,  310 
muscle,  310 
Stapes,  221,  310 
muscle  of,  310 
Stars,  lens,  302 
Stem  buds,  of  lung,  116 
Stenson’s  canal,  89 
Sternal  bars,  216 
Sternum,  216 
cleft,  222 

I Stoerck’s  loop,  143 
Stomach,  104 
pig.  358,  382 
, rotation  of,  104 
Stomodeum,  85 
Stratum  corneum,  231 
germinativum,  231 
granulosum,  20,  23,  231 
lucidum,  231 
Streak,  primitive,  39,  40 
chick,  314 

Stroma,  of  ovary,  156 
Stylo-hyoid  ligament,  221 
Styloid  process,  219,  221 
Subcardinal  vein,  199 


43° 

Subclavian  artery,  i88,  189,  191,  194 
vein,  199,  200 
Sublingual  gland,  98 
Submaxillary  gland,  98 
Substance,  anterior  perforated,  267 
white,  249 

Succenturiate  placentas,  67 
Sudoriferous  glands,  234 
Sulcus,  central,  272 
cerebral,  273 
coronary,  175 
interventricular,  183 
limitans,  248 
spiral,  307 

terminalis,  of  tongue,  96 
Superfetation,  28 
Superior  concha,  297 
ganglion,  285 
Supporting  cells,  293 

elements  of  neural  tube,  242 
of  ganglia,  244 
tissues,  206 

Supracardinal  vein,  194 
Sujnarenal  artery,  192 
gland,  290 
accessory,  291 
anomalies  of,  291 
vein,  199,  200 
Supratonsillar  fossa,  100 
Surface  area,  increase  in,  82 
vSuspensory  ligament,  of  lens,  302 
Sustentacular  cells  (of  Sertoli),  16,  17,  154 
Suture,  21 1 
Sweat  glands,  234 
Swellings,  arytenoid,  115 
labio-scrotal,  163 
Sylvian  fissure,  272 
Symijathetic  ganglia,  242,  287 
nervous  system,  287 
trunks,  287 
Synarthrosis,  21 1 
Synchondrosis,  21 1 
Syndactyly,  81,  222 
Syndesmosis,  21 1 
vSynovial  membrane,  21 1 
System,  central  nervous,  237 
digestive,  85 
integumentary,  230 
lymphatic,  203 
muscular,  223 
peripheral  nervous,  274 
respiratory,  114 
skeletal,  206 
sympathetic,  287 
urogenital,  135 
vascular,  185 

Tables,  of  bone,  209 
Tactile  corpuscle,  292 
Tffinia,  257 
Tail,  73,  80 
fold,  71 
gut,  86 

of  caudate  nucleus,  265 
of  spermatozoon,  15 
Tarsal  glands,  304 
Tarsius,  23,  27,  34,  36,  42 
Tarsus,  222 


INDEX 

I Taste  buds,  97,  292 
' cells,  293 
Teeth,  89 

ameloblasts  of,  91 
anomalies  of,  95 
cement  of,  93 
decidual,  90 

periods  of  eruption  of,  93 
dental  lamina  of,  89 
papilla  of,  92 
pulp  of,  92 
sac  of,  93 
dentine  of,  92 
I enamel  of,  89 
I milk,  90 

periods  of  eruption  of,  93 
odontoblasts  of,  92 
permanent,  93 
of  vertebrates,  94 
periods  of  eruption  of,  94 
Tela  chorioidea,  260 
] Telencephalon,  251,  263 
: commissures  of,  268 

' Telolecithal  ova,  14 
Telophase  of  mitosis,  5 
Temporal  bone,  219 
j lobe,  270 

operculum,  272 
j Tendon,  207 
I Tensor  tympani,  310 
Teratomata,  156 
Term,  66 

Terminal  ganglia,  288 
nerve,  279 
I nuclei,  257 

, ramus,  277 

' sinus,  chick,  227 

Testis,  153 

anomalies  of,  156,  163 
appendix  of,  157 
concealed,  163 
cords,  153 
descent  of,  16 1 
differentiation  of,  153 
gubernaculum  of,  16 1,  162 
intermediate  cords  of,  153,  154 
interstitial  cells  of,  154 
ligament  of,  161 
mediastinum,  154 
tubuli  contorti,  154 
septula,  154 
recti,  154 

tunica  albuginea  of,  153 
vaginalis  of,  163 
I Tetrad,  17 
Thalamus,  260 
j Thebesian  valve,  179 
I Theca  folliculi,  20 
Theory  of  concrescence,  39 
Thoracic  duct,  203 
Thoraco-abdominal  muscles,  226 
Thorax,  muscles  of,  226 
Thymic  corpuscle,  10 1 
Thymus  gland,  100 
Thyreo-cervical  trunk,  191 
Thyroglossal  duct,  103 
Thyroid  cartilage,  116,  221 
^ follicles,  103 


INDEX 


431 


Thyroid  gland,  103 
anomalies  of,  103 
Tibia,  222 
Tibial  nerve,  278 
veins,  200 
Tissue,  6 
adipose,  207 
areolar,  207 
connective,  206 
white  fibrous,  206 
differentiation  of,  206 
elastic,  207 
lymphoid,  204 
marrow,  210 
nephrogenic,  142 
nervous,  237 
reticular,  206 
supporting,  206 
Titles  for  reference,  ii 
Tomes,  dentinal  fibers  of,  92 
Tongue,  96 

anomalies  of,  97 
muscles  of,  97,  229 
papillae  of,  96,  97,  293 
Tonsil,  100 
lingual,  100 
palatine,  100 
pharyngeal,  100 
Tonsillar  fossa,  100 
Trabeculae  carnae,  184 
Trachea,  114,  116 
anomalies  of,  118 
pig.  358.  381 

Tract,  descending,  of  fifth  nerve,  257,  283 
olfactory,  266 

Tractus  solitarius,  257,  285,  287 
Tragus,  312 

Transposition  of  viscera,  118 
Transverse  colon,  108 
foramina,  215 
mesocolon,  125 
Triangular  ligaments,  126 
Tricuspid  valve,  183 
Trigeminal  nerve,  283 
Trigone,  of  bladder,  147 
Trochlear  nerve,  282 
Trophectoderm,  33,  54,  62 
Trophoderm,  54,  56,  57,  58,  62 
True  chorionic  villi,  54,  57 
Trunk,  80 

anomalies  of,  80 
muscles  of,  226 
Tubal  pregnancy,  67 
Tube,  auditory,  100,  310 
digestive,  103 
Eustachian,  100 
neural,  36,  237 
Pfliiger’s,  156 
uterine,  157 
Tuber  cinereum,  261 
Tubercle,  Darwin’s,  312 
genital,  163 
Muller’s,  147,  157 
satyr,  31 1 
Schwalbe’s,  31 1 
Tuberculum  impar,'96 
Tubules,  collecting,?  139,  141 
distal  convoluted,  142 


Tubules,  mesonephric,  137 
transformation  of,  156 
proximal  convoluted,  142 
secretory,  139 
uriniferous,  142 
Tubuli  contorti,  154 
recti,  154 

Tunic,  vascular,  of  lens,  303 
Tunica  albuginea,  of  ovary,  155 
of  testis,  153 
externa,  20 
interna,  20 
vaginalis,  163 
Turbinals,  297 
Twins,  and  twinning,  42,  67 
origin  of,  43 

Tympanic  cavity,  100,  310 
membrane,  31 1 


Ulxa,  221 
Ulnar  artery,  194 
nerve,  278 

Ultimobranchial  body,  102 
Umbilical  arteries,  185,  193 
cord,  56 
hernia,  107,  109 
ligaments,  lateral,  203 
middle,  147,  203 
veins,  185,  194 
vessels,  47,  54 
Umbilicus,  54 
Unguiculates,  10 
Ungulates,  10 

Unmyelinated  nerve  fibers,  245 
Upper  jaw,  78 
Urachus,  147 
Ureter,  139 
Urethra,  146,  147,  164 
Urethral  folds,  163 
groove,  163 
membrane,  163 
Urinary  organs,  135 
system,  chick,  352 
Uriniferous  tubules,  142 
Urogenital  fold,  135,  137,  149 
membrane,  85,  146 
sinus,  85,  145 
system,  133 
chick,  352 
pig.  360,  383 

Uterine  glands,  of  pregnancy,  61 
tubes,  137 

Utero-vaginal  anlage,  138 
Uterus,  137 

anomalies  of,  139 
bicornis,  139 
cervix  of,  138 
during  menstruation, '59 
pregnancy,  60 
fetalis,  139 
fundus  of,  138 
gross  changes  in,  67 
growth  of,  139 
infantalis,  139 
ligaments  of,  160 
planifundus,  139 
Utricle,  prostatic,  137 


432  INDEX 


Utriculus,  306 
Uvula,  89 

Vagina,  137 

anomalies  of,  159 
masculina,  157 
Vaginal  sac,  162 
Vagus  ganglia,  accessory,  286 
nerve,  285 
Vallate  papillae,  97 
Valves,  atrio-ventricular,  183 
bicusiiid,  184 
colic,  108 
Eustachian,  179 
mitral,  184 

of  coronary  sinus,  1 79 
of  inferior  vena  cava,  i 79 
of  sinus  venosus,  177,  179 
semilunar,  181 
Thebesian,  179 
tricuspid,  183 
Vascular  system,  185 

chick,  321,  329,  339,  352 
pig,  361,  384 
Vasculogenesis,  i6g 
Vegetal  pole,  14 
Veins,  anomalies  of,  201 
axillary,  200 
azygos,  199 
basilic,  200 
border,  200 
brachial,  200 

cardinal  anterior,  187,  197 
common,  187,  197 
posterior,  187,  199 
cephalic,  200 
cerebral,  197 

common  cardinal,  187,  197 
femoral,  200 
gluteal,  200 
hemiazygos,  199 
hepatic,  196 
common,  200 
iliac,  200 
innominate,  197 
intercostal,  200 
jugular,  199 
lumbar,  200 

mesenteric,  superior,  194 
oblique,  of  left  atrium,  197 
of  arm,  200 

of  caval  mesentery,  200 
of  extremities,  200 
of  leg,  200 
ovarian,  200 
jjrecardinal,  187,  197 
portal,  194 

postcardinal,  187,  199 
primary  head,  197 
pulmonary,  118,  181 
renal,  200 
saphenous,  200 
spermatic,  200 
subcardinal,  199 
subclavian,  199,  200 
supracardinal,  199 
suprarenal,  199,  200 
tibial,  200 


Veins,  umbilical,  185,  194 
vena  cava,  inferior,  199,  200 
superior,  197 
vitelline,  186,  194 
chick,  330,  340,  352 
pig,  363,  386 
Velum,  medullary,  259 
Vena  cava,  inferior,  199,  200 
superior,  197 
Ventral  aorta,  185 
arteries,  192 
mesentery,  125 

differentiation  of,  1 25 
mesocardium,  126 
pancreas,  1 1 2 
Ventricle,  fifth,  270 
fourth,  253,  256 
lateral,  232,  263,  270 
of  heart,  1 74,  183 
of  larynx,  1 16 
third,  232,  261 
Vermiform  process,  108 
Vermis  cerebelli,  238 
Vernix  caseosa,  31,  231 
Vertebrae,  214 
anomalies  of,  222 
arch  of,  214 

articular  process  of,  215 
body  of,  214 
chondrification  of,  214 
epiphysis  of,  213 
ossification  of,  213 
transverse  process  of,  213 
variations  in  number,  222 
Vertebral  arteries,  190 
Vertebrate  body  plan,  10 
groups,  10 

head,  segmentation  of,  229 
Vertebrates,  10 
Vesicle,  auditory,  305 
blastodermic,  34 
brain,  primary,  246 
cervical,  100 
lens,  297 
optic,  231,  297 
otic,  303 
seminal,  137 

vesico-urethral  anlage,  146 
Vesicular  follicles,  20,  22,  156 
Vestibular  anlage,  305 
glanglion,  280 
glands  (of  Bartholin),  149 
membrane,  309 
Vestibule,  138,  166 
of  mouth,  87 
Villi,  anchoring,  37,  62 
free,  62 

of  chorion,  49,  34,  57 
of  intestine,  107 
Visceral  mixed  nerves,  283 
musculature,  224 
nerves,  273 
pleura,  118 
Vitelline  arteries,  186 
circulation,  186 
membrane,  14 
veins,  186,  194 
Viteilo-umbilical  trunk,  186 


INDEX 


433 


Vitreous  body,  of  eye,  302 
Vocal  cords,  1 16 
Vomer,  220 

Vomero-nasal  organ,  279,  296 
Waters,  51 

Weight  of  human  embryos,  68 
White  blood  cells,  17 1.  See  also  Leucocytes. 
commissure,  249 
fibrous  connective  tissue,  206 
rami,  287 

substance,  of  cord,  249 
Winslow’s  foramen,  121 
Wirsung,  duct  of,  113 
Witch  milk,  236 
Wolffian  ducts,  139 

JV-CHROMOSOME,  29 

Xiphoid  process,  216 
cleft,  222 
28 


F-chromosome,  29 
Yellow  bone  marrow,  210 
Yolk,  12 
sac,  44 
birds,  44 
mammals,  46 
man,  51 
reptiles,  44 
stalk,  45,  52 

anomalies  of,  52 
Yolk-sac  placenta,  46,  48 


Zeiss’  glands,  304 
Zona  pellucida,  14 
Zone,  ependymal,  238 
mantle,  238 
marginal,  238 
Zonula  ciliaris,  302 
Zuckerkandl’s  bodies,  289 
Zygomatic  bone,  220 


1 


